Do you love natural stone? Me too. Fans of natural stone countertops, tiles, fireplaces, walls, and building stone are natural allies to geologists. We all share a similar zeal for a glimmer of garnet and the sexy sparkle of marble. The two disciplines have different ways of organizing and thinking about stone, which makes sense because we’re interested in different things. Geologists study rocks to learn more about what happened in Earth’s past. Regular people appreciate rocks because they’re useful, practical, and beautiful. Nonetheless, a bit of geology can shed light on why or where we’d want to use a given stone. Geology also helps us appreciate that every slab of stone offers a little glimpse into deep time and the dramatic forces that shape the planet.

This article touches on 13 different types of natural stone, with links to each type to explore further.

Organizing natural stones into categories

You learned this in 8th grade: Geologic categories of stone

Geologists, like all scientists, love to put things in categories. We group rocks by the process that formed them. Did a rock erupt from a fiery volcano, or take shape in a placid river bed?

Making more useful categories: Grouping stone by their properties

I’ll admit that geologic categories aren’t all that helpful for the way natural stones are used. Instead, we can group stones by the properties that matter to us, like how they hold up in the places we use them.

Here’s an overview of rock types sorted by hardness and their ability to withstand an onslaught of marinara sauce.

Note the empty category of hard stones that are damaged by acids. It turns out that hard minerals also tend to be unaffected by household acids. That’s just a coincidence of mineral chemistry, but it’s convenient for those looking for a hard, resistant natural surface.

When we talk about acid resistance, we’re referring to things like vinegar or citrus. Strong acids like oven cleaner and some bathroom cleaners will damage almost any stone, so keep those away from your stone, or better yet, use less toxic means to clean your home.

Relationships between different types of stone

In the world of geology, all stones are related to each other. Over deep spans of time, any rock can turn into a whole new rock if it gets melted, squeezed, uplifted, or eroded. In fact, that’s exactly what’s been happening all through Earth’s history. Understanding the relationships between different stones can make it easier to see why some share similar traits. It also helps you appreciate the events that gave rise to all those beautiful slabs in the showrooms.

Sedimentary rocks turn into metamorphic ones

Limestone forms in shallow, warm oceans and coral-rich beaches. It’s made of shells, shell fragments, and dissolved shells. Limestone can get buried and heated to taffy-like consistency, wherein it turns to marble. Both stones are made of the same mineral – calcite – but in marble’s case the calcite grains have been crystallized together, making the stone less porous. Marble’s distinctive grey streaks are clay layers from the original limestone that got heated and swirled.

The relationship between sandstone and quartzite follows a similar theme. Sand grains gather on beaches, sand dunes, and riverbanks. Layers of sand get buried and pressed together, forming sandstone. If sandstone gets shoved down deep and compressed even further, the sand grains fuse together to become quartzite. As described in the Deep Dive Into Quartzite article, this process is a gradual one. This means there are many gradations of sandstone and quartzite, ranging from highly porous sandstone to bombproof crystalline quartzite. The more deeply a stone is buried, the more tightly compacted it will get. The porosity of a stone translates into its ability to shrug off stains, and this is something that buyers can evaluate as they shop for different stones.

Bluestone is a variety of sandstone. It formed as rivers flowed off a former mountain range along the eastern seaboard. As 400,000,000 year-old rivers wound their way through the landscape, they left behind pockets of sandstone in Pennsylvania and southern New York. Because the sandy deposits occurred in small, scattered areas, the quarries were small, too, setting the stage for generations of family-run quarrying operations throughout the region.

Dialing up the heat on slate, schist, and gneiss

This trio of rocks shows what happens if you crank up the thermostat on a stone. The predecessor to all of these stones is shale, which is compressed clay and is decidedly un-sexy. But add a little heat and pressure and those unremarkable clay particles start to grow and strengthen and the rock turns into slate. Unlike shale, slate is durable – and is workable into tiles, shingles, and of course, old-school blackboards.

If the stone gets hotter, the clay particles morph into mica and the rock takes on a subtle sheen. This is called phyllite, but in commercial terms phyllite is usually sold as slate. More heat begets even larger mica grains, and the stone becomes schist, which is known for its glittery look. Schists with small mica grains are preferable to stones with large chunks of mica, because the latter can be weak and literally flaky.

Adding even more heat and pressure will make the stone separate into bands of light and dark minerals. Striped or banded patterns are the hallmark of gneiss. The patterns can be calm or bold, straight or swirled. For commercial purposes, gneiss is usually classified as granite, because it’s made of the same minerals, and shares similar properties and colors.

If the stone gets hotter still, it will start to melt. In some slabs of gneiss you can see melted blobs of quartz, showing the stone was right on the edge of becoming liquid again. If the whole thing melts, then you’ll end up with granite, an igneous rock.

The many colors of granite

Granite means many things. It’s a catch-all category that’s often used to describe any hard, crystalline stone. In geology, granite is one specific thing: an igneous rock that is coarse-grained and overall light-colored. But in the parlance of the natural stone industry, the definition of granite is expanded to include all igneous rocks, as well as many metamorphic rocks like gneiss and schist.

Geologists classify igneous rocks by the size of the crystals and the types of minerals. And while you will definitely sound smart if you casually inquire if your local slab yard has any quartz monzonite or granodiorite, that’s not necessary. Most igneous rocks are quite similar to each other, despite their different colors and patterns. We can just stick to calling them all granite. (Just please don’t tell my geology friends I said this!)

Basalt is one type of igneous rock that is famous for erupting out of volcanoes, like the spectacular lava flows from Kilauea we saw last spring and summer. Basalt also makes up the entire ocean crust (which itself is volcanic, betcha didn’t know that!) and forms oceanic islands like Hawaii and Iceland.

 

Solid rock from liquid water

Onyx and travertine are variations of the same stone. They come about from mineral-laden water, like you’d find at the mouth of a hot spring. Both are made of calcite, the same mineral that’s in limestone and marble. Travertine has a lacy pattern from the way the water flows in little rivulets away from the mouth of a hot spring. Onyx can form from either hot or cool water, and is less porous than travertine. Onyx is beloved for its smooth layers and gem-like colors that are especially glorious when backlit.

Last but not least: The oddball stone that doesn’t fit into any categories

It’s true with people and it’s true with stones, too. Not everyone fits into a tidy category. Soapstone, for instance, is basically nothing like other rocks. It manages to be soft, yet dense. You can scratch it with your fingernail, but you can’t stain it or burn it, no matter how careless you are. Soapstone is more like a family of stones rather than one specific thing, but it is cool stuff, and I bet it will surprise you.

Geologists and natural stone aficionados agree

Natural stone offers a lot to love—there’s zero doubt about that. Take a moment to learn about the history and characteristics of your favorite stone, and I bet you’ll appreciate it even more.

More from the Geology Series

While browsing the slab yard or showroom, each type of stone seems to possess a distinct personality. Glossy, polished granite is the crisp tailored suit, while marble is like flowing linen—a little wrinkled but always in style. And soapstone? Without a doubt, it’s the flannel shirt of natural stone. Soft, touchable, and always comfortable. You can’t even walk by a slab of soapstone without stopping to pet it.

Soapstone enjoys a cult-like fan base: “I love the organic-ness of it,” writes one devotee on Houzz. But it’s not for everyone. “I find these photos rather sobering,” laments another commenter after viewing images of the patina that soapstone can acquire.

 

Yes, Soapstone is Soft

“All stones have benefits and disadvantages,” says Anthony Lacour of M. Teixeira. “There’s no such thing as a perfect surface.”

As the branch manager at M. Teixeira’s Denver office, Lacour has been working with this iconic stone for a dozen years. He suggests the best way to get a feel for soapstone’s hardness is to experience it personally. “Let people test for themselves,” he says.

Hardness of most commercial soapstones ranges from 1 to 4, but Lacour has found that the exact Mohs number is not as important as just getting a sense for it. “Use a fingernail and a key as your hardness indicators,” he advises.

Soapstone’s softness might be a dealbreaker for some—it comes down to knowing yourself and recognizing if a scratch in the stone is going to ruin your day, or how long it will take you to forgive a family member who accidentally drops a frying pan on the countertop.

Lacour explains the upside: “The good thing is that you can fix it yourself.”

A fresh coat of mineral oil will render small scratches invisible. Larger marks can be removed with sandpaper. Lacour recommends 120-220 grit for most scratches; if you prefer a shinier finish, you can follow up with 400-500 grit. Then re-oil the surface to restore the luster, and you’re all set.

Even a gouge is fixable—it can be filled with a mixture of stone chips and epoxy. This can be tackled by a professional or by a seasoned DIYer.

Steve Schrenk is the digital media director for Polycor, and he has worked with stone as a sculptor, fabricator, and designer. He describes soapstone as “very user friendly for self-repair,” noting that soapstone appeals to the type of person who “likes to roll up their sleeves and do it themselves.”

A Houzz commenter sums it up best, “What I love love love the most about soapstone: If you are a DIYer at heart (or a control freak, like I am), then the self-maintenance of the counter is just wonderful.”

 

And Now for the Good News: Soapstone’s Superpowers

Once you’ve come to terms with soapstone’s softness, you can revel in its strengths. By and large, the stone is remarkably resistant to staining or damage from acids. The density of the stone makes it practically impervious. That smoldering casserole you left in the oven while binge-watching Better Call Saul? Put that right there on the stone, thank you very much.

These superpowers were not bestowed on soapstone by any magical process. It comes down to the properties of the minerals in soapstone. Soapstone’s primary ingredient, talc, is chemically inert, which is why soapstone is generally unaffected by acids and alkalines. Talc is also hydrophobic, meaning it quite literally repels water.

The mineral magnesite is another major component of soapstone, and it has the ability to retain heat. The properties of magnesite allow soapstone to work so well as an insulator or a fireplace. The stone is capable of absorbing heat and then slowly radiating it off, providing the long, sustained warmth so appreciated in cold climates. Lastly, soapstone is a dense arrangement of small minerals, with minimal pore space that might allow water to seep in. Low porosity means that soapstone will not absorb liquids nor harbor bacteria—a perk for the germaphobes among us.

Schrenk sums up the yin and yang of soapstone: “People tend to latch on to the scratching aspect of soapstone,” he says. But they may be “missing out on some of the other aspects of the stone” that make it so appealing.

 

Soapstone’s Quirky Geology

Soapstone is an unusual type of metamorphic rock. Most metamorphic rocks are heated and squished versions of their former selves. Limestone becomes marble. Sandstone turns to quartzite. But soapstone doesn’t abide that simple recipe. Soapstone comes about from a series of chemical reactions that take place deep underground. Hot groundwater carries dissolved minerals from one rock to another, allowing a mingling of ingredients that begets entirely new minerals. In some cases, soapstone is a derivative of dolomite or dolomitic marble (like Super White). In others, soapstone comes from ultra-deep, ultra-dense rocks from Earth’s mantle. In this latter case, serpentine is also formed, and hence, soapstone and serpentine can often be found side-by-side in a quarry or outcrop.

Soapstone’s signature soapiness comes from the mineral talc. Talc is a member of the mica family, and is made up of thin flakes. The flakes are held together by a very weak type of bond, which allows the layers to easily slide against each other, giving the stone a slippery feel.

Talc is the softest mineral on Earth, with a hardness value of 1. Soapstone always contains some talc, but the amount varies, and therefore so does the overall hardness of soapstone. Stones with high talc content are called steatite, and are useful for carving. Commercial soapstone slabs generally have 30% to 50% talc, with the remainder of the stone being chlorite, magnesite, amphibole, and other minerals. In general, the lower the talc content, the greater the hardness.

Geologically, soapstone is a family of stones rather than one specific thing. There’s broad variation in the types of minerals present and their proportions. This is all the more reason to thoroughly investigate potential stones and to work with reputable dealers and fabricators.

If a stone looks sort of like soapstone but can’t be easily scratched with a pocketknife, it’s likely serpentine rather than soapstone. Virginia Mist and Jet Mist are granites that resemble soapstones, but are much harder. When in doubt, use the diagram below and read up on how to tell green stones apart.

 

Evaluating the Hardness of Various Soapstones

Soapstone really is different from most other types of stone. What better way to explore this than to try a battery of tests and see how it performs. M. Teixeira sells a sample kit of various soapstones to allow customers to experience a range of soapstone colors, textures, and hardnesses. I used a set of Mohs hardness picks to get up close and personal with the hardness of 12 soapstones. I tried to scratch different minerals in the stone with picks of various hardness values. I also used my fingernail and a pocketknife so that I could compare common items with Mohs numbers.

The results varied depending on how much talc the stone contained and what other minerals were present. While all the samples fell into a range between 1 and 4, they were different from one another, and even a single stone could have different hardness values in different places.

A summary of soapstone hardness is given in the table below, and the stones are arranged roughly from softer to harder.

 

Testing Acid Resistance

After I checked out the hardness of the 12 soapstones, I moved on to acid tests. I put a 10% solution of hydrochloric acid on every sample, and also used standard white vinegar to check for etching. I left the acids there for at least a half hour, before scrubbing the stones with soap and water and inspecting them for damage.

Several stones had a slight bubbling reaction with hydrochloric acid. This is expected because soapstone sometimes has calcite veins, and/or it can contain magnesite, which will have a very subtle reaction to acid. Interestingly though, 11 of the 12 stones showed no damage, etching, or staining from the hydrochloric acid, even though the acid initially looked like it had stained some of the stones. But it washed away and left the stone unaffected. Similarly, vinegar left no mark whatsoever on 11 of the 12 stones.

One stone, Stormy Black, showed a faint etch mark where vinegar was left puddled up for several hours, and the hydrochloric acid left a stippled white pattern on the black stone when left for an hour. This stone appears to be somewhat susceptible to acids, and likely has more magnesite than the other samples. The dark color and fine texture of Stormy Black may also make the changes appear more noticeable.

Aside from that, the other 11 samples looked brand “new” (while appreciating the fact that they are indeed several million years old!).

 

Oiling or Waxing Enhances the Finish of Soapstone

Soapstone can have two distinct looks, depending on the finish. It can be left untreated and will attain a soft grey color and a matte finish. Or, the surface can be oiled or waxed, which will darken the color and impart a satiny luster. “You get multiple looks from the same stone,” says Lacour.

To oil, or not to oil: that is a frequent dilemma among soapstone owners. Light oil like mineral oil is easy to wipe on but will fade fairly quickly. Wax can also be used as a surface treatment. It requires more effort to apply, but it lasts longer. An oil/wax blend strikes a happy medium between the two.

One needn’t agonize over this decision, because it’s always reversible. Thanks to soapstone’s density, the oil simply sits on the surface. It doesn’t penetrate the stone. If you change your mind and decide you don’t like the oiled look of your soapstone, you can scrub most of it off right away, or simply wait for it to disappear on its own.

One minor caveat to the oiled vs. naked finish is the color of the seams. During installation, the epoxy in the seams will be dyed to match the stone. But since the color of the stone depends on whether it’s oiled or not, you could end up with a slight mismatch. When in doubt, savvy soapstone owners suggest going with a lighter color in the seams, because the seam can be darkened more easily than it can be made lighter.

 

Best Uses for Soapstone

Around the world, soapstone has been shaped into cooking pots, utensils, vessels, and sculptures for thousands of years. Soapstone’s workability made it especially useful during the Bronze Age, when molds were carved out of soapstone and filled with molten metal.

Today, soapstone is a beloved material for wood stoves and fireplaces. At the Bridger Bowl ski area in frigid Montana, a soapstone stove is the central feature in a slopeside warming hut. The stove is thoughtfully placed in the middle of the room, beckoning to visitors and easing the woes of chilly children. Once heated, the stone gives off warmth long after the fire goes out. Even after sitting idle all night long, the soapstone is still warm the next morning.

Soapstone is well known as a worktop in laboratories, where its chemical inertness and heat resistance allow it to withstand all manner of abuses.

These same traits make soapstone an enviable material in the kitchen, where it’s commonly used for countertops, islands, or sinks. Unlike many types of natural stone, soapstone cannot be polished to a glossy shine. Instead, it has a soft, warm glow and rounded edges that help a space feel comfortable rather than imposing. “Our kitchen looks loved! It looks lived in! People who eat together and tell stories and make each other laugh and spill their drinks live here!” writes a fan of soapstone on Houzz.

Both Lacour and Schrenk agree that soapstone fills a distinct niche in the industry. Soapstone’s grey tones and honed finish are on-trend nowadays, but soapstone won’t ever go out of style. While some stones draw attention to themselves, “soapstone’s subtle colors plays well with other elements of design,” explains Schrenk. “It fits a 100 year old building or new construction,” observes Lacour. Schrenk echoes the same sentiment: “It’s modern use of a traditional material.”

 

Alberene – Virginia’s Soapstone Quarry

Not far from Charlottesville, Virginia sits a large soapstone deposit and an active quarry that dates back to 1883. Over the years, Alberene soapstone was destined for a variety of applications, ranging from industrial uses of talc in tires and roof shingles, to practical objects like sinks, bed warmers, and griddles. Polycor purchased the Alberene quarry 5 years ago, and they’ve been pleasantly surprised by the enduring popularity of soapstone. “It’s surpassed everyone’s expectations,” says Schrenk. Alberene’s most popular soapstone is called Church Hill, literally named because of a hilltop church near the quarry. Nowadays, the quarry mostly produces slabs destined to become countertops, tiles, pavers, treads, wall caps, and fireplace inserts. “There’s a huge surge right now, especially for countertops,” says Schrenk.

The uptick in demand is easy to appreciate. Alberene soapstone is local, natural, and has “a tactile quality” that customers are drawn to, explains Schrenk. He finds that once customers learn what sets soapstone apart from other stones, “they sing its praises.”

 

Is Soapstone Right for You?

Choosing stone is always a personal decision, but even more so with soapstone. Only you know your situation and squeamishness, and Lacour’s advice to test the stone yourself rings true. Thankfully, soapstone dealers typically provide samples for customers to work with, so you can bring pieces home and try out your own scratching, etching, and staining tests. (You can also take our Stone Personality quiz.)

This bit of online advice nails it: “I’m looking forward to the aging and patina. But it would be sad if someone who wasn’t up for it spent a lot of money and had their heart broken.”

As with every material, the more you learn about it and the better you understand your own needs, the easier it will be to narrow down your choices.

If you’re curious about soapstone, spend some time interacting with it and see what you think. I found myself falling more in love with the samples the more I played with them. Another Houzz commenter said it best: “To those of you considering soapstone, do not be afraid!”

 

More from the Geology Series

Quartzite might just be the hottest natural stone at the moment. If quartzite were on Instagram, it would have thousands of followers, and as with any other celebrity sensation, people would wonder in hushed tones, “Can that be real?” “Looks too good to be true.” Or, “I’m not sure if I can trust it.”

With its marble looks and granite toughness, sometimes quartzite does seem too good to be true. And because of widespread mislabeling, sometimes it really isn’t true. One of the downsides of quartzite’s popularity is a tendency for a quartzite label to be put on stones that aren’t actually quartzite. Thus, the burden falls on local dealers, fabricators, and consumers to try to figure out if a given quartzite is the real deal or if it’s one of many imposters.

Overall, this isn’t a terrible thing. Quartzite is pushing people to look beyond aesthetics and use the properties of a stone to determine what it is and how we should use it. Looks can be deceiving (and alluring!) but a stone’s inherent traits are something we can rely on, every time.

Case in point, the most common mixup around quartzite is that marble is sometimes incorrectly called quartzite. Thankfully, this question is easily resolved with hardness and etching tests. Quartzite is hard; marble is soft. Anyone with a glass tile can easily tell the difference. The details are spelled out in The Definitive Guide to Quartzite.

Sandstone and quartzite are similar, but different

Lately I’ve been seeing a new wrinkle: sandstone being labeled as quartzite. This one is trickier to resolve, because sandstone and quartzite are made of the same mineral – quartz – and they have similar properties. Both sandstone and quartz have a hardness of 7 and will easily scratch glass. Neither will be etched by common acids.

So how do you tell sandstone and quartzite apart? And does it even matter?

The bottom line is, yes, you can tell quartzite and sandstone apart by examining their textures. And, yes, it does matter in terms of porosity. Moderately porous stones are perfectly usable, but should be sealed in certain applications (such as countertops or flooring) and may be more prone to staining. Given the choice between an accurate name or a generalized, semi-true label, we should always go with the former. As always, the key is to evaluate what stone you have and what its properties are before you commit to using it.

Let’s delve into the details, shall we?

The sandstone – quartzite continuum

Most likely you know by now that sandstone is a sedimentary rock. Sandstone starts out as humble beach sand. Eventually, sand gets buried and compressed by overlying layers, and minerals carried by groundwater act as glue that binds the sand grains together into solid rock. All of this and more is described in our article about sandstone.

Quartzite picks up where sandstone leaves off. It’s a metamorphic rock – one that’s been baked into an extra-tough stone by the heat and pressure that only comes from deep burial way down in Earth’s crust. Such events are usually brought about by tectonic collisions, where continents grind into each other.

Quartzite isn’t melted sandstone. It’s sandstone that is fused together so tightly that the sand grains lose their individual identities. The minerals crystallize together into a dense fabric of quartz crystals. The deeper and hotter the stone gets, the more tightly it’s fused.

Here is the key point: The heat and pressure that turn sandstone to quartzite is not a definitive, black-and-white occurrence. It’s a gradual process, with subtle differences occurring all along the spectrum. There is no exact moment that sandstone becomes quartzite. It’s similar to the way that colors blend from one shade to the next. When does Royal Blue become Navy Blue? It’s hard to pin down, exactly. Because there is a range of quartzites and sandstones, it’s wise to assess each stone as an individual, rather than relying on broad categories or stone names as the last word on how a given stone will behave.

Five shades of grey: Organizing sandstone and quartzite by their properties

To help make these ‘shades of quartzite’ easier to understand, we can break down the sandstone-quartzite continuum into 5 categories. These range from porous sandstones, which are only loosely bound together, all the way to crystalline quartzite, which got so hot it nearly melted. Then there’s what I’ll call “mixed stone” quartzites, which are on the borderline between quartzite and other metamorphic rocks like schist and gneiss. These stones contain some quartzite, along with other minerals and rock types. In general mixed stone quartzites are darker in color than regular quartzites, and they have swirls and blobs of different rock types blended together. The table below outlines the 5 types of sandstone and quartzite.

Ways to distinguish sandstone and quartzite

 So, we’ve established there is a range of different stones in the sandstone/quartzite family. How do you know which is which?

Here are three ways you can determine what type of sandstone or quartzite you’ve got. Each of these is explained in detail below.

1. Inspect the texture – is it sandy and granular? Or crystalline and glassy?
2. Look for signs of cross bedding, which is common in sandstone but absent in highly-metamorphosed quartzites.
3. Test the porosity by setting the stone in water and observing if the stone soaks up water.

Let’s explore each of these geo-detective methods. Oh, and if your eighth-grader is looking for a novel science fair project, this would be a worthy one. Who knows, maybe this could spur your child to pursue the greatest career ever? (By that I mean geology, of course.)

1. Do you see sand grains?

The key difference between sandstone and quartzite is whether the stone is made of individual sand grains, or has been recrystallized to the point where the grains became tightly interlocked.

You can tell the difference by looking at the stone. In most cases, you need a magnifier of some sort. Bring the stone to a well-lit area. Look at the surface of a slab, not the broken edge. Look as closely as you can, and to try to see the texture and structure of the stone. Use the photos below as examples.

2. Do you see cross beds?

As water or wind flows across sand, it makes a distinctive pattern called cross bedding. Cross beds look like repeating, curved layers. They can be so big they take up the whole slab, or they can be just a few inches across. Sometimes the stone has cross beds everywhere, and in other cases, they occur in just a few places. Cross beds can have several different styles, and it takes a while to tell them apart from regular layering.

Cross bedding is a good news/bad news situation. The good news is that cross bedding is an easy giveaway that the stone is not marble. Yay! But that also means it’s a sandstone or a grainy quartzite and is likely to have higher porosity compared to a crystalline quartzite. When a stone undergoes a high degree of metamorphism, it fully erases cross beds.

Bottom line – if the stone has cross beds, it’s important to test the porosity because the stone was not subjected to a super high degree of metamorphism.

3. Does it absorb water?

The more compressed a stone got during its formation, the lower its porosity will be. An easy way to check the porosity is to take an unsealed chunk of stone and let one edge of it sit in water for about 15 minutes.

Take the stones out of the water and see if you can observe water wicking up into the stone, above the level of the water that the stone had been sitting in. On the sample of White Macaubus below, you can clearly see the water has wicked up into the stone. This shows that the stone is porous.

Wild Sea sandstone, surprisingly, appears to be less porous than White Macaubus quartzite. Moisture only soaked up to the water line, but no higher. This tells us that Wild Sea has particularly solid cement holding those sand grains together.

On this example (Taj Mahal), the stone doesn’t even look wet. This stone has low porosity. That’s what we’d expect for a crystalline quartzite, which has been subjected to intense pressures and high temperatures that welded all the mineral grains together into one solid chunk.

The fact that some quartzites are porous need not be a dealbreaker. But these stones need to be sealed, which will fill in the pores on the stone’s surface. Be sure to thoroughly seal all worked edges of the stone, not just the tops, and re-seal it from time to time. Even the best sealers may require multiple applications for full effectiveness on porous stones. On the positive side, modern sealers are very resilient and should last for many years under normal use.

Summary

This has been a detailed look at the nitty-gritty of quartzite, so if you’ve made it this far, congratulations! Here are a few points to hammer home.

• Different stones form in different zones in the Earth’s crust. The depth of burial and/or the type of mineral cement determine how porous a stone is.
• There’s a gradient of stones along a continuum from sandstone, to intermediate quartzite, to crystalline quartzite. Some stones are a mixture of quartzite and schist and/or gneiss.

As always, don’t take my word for this. Stone names are variable, and stones themselves are variable. The properties will tell you so much more than the names. Always test potential stones before committing to them, because even stones in the same family can have very different physical characteristics. Learn as much as you can, and let that guide your decision about which stones to work with. Sandstones and quartzites are fabulous, durable stones with a range of colors, patterns, and potential uses. The better we understand them, the more we can enjoy them.

 

More from the Geology Series

This is the stone that started it all for me. In the throes of a kitchen remodel, I ventured to the local stone yard and worked my way down a tantalizingly long aisle of slabs and remnants. My husband was wise to let me make the first cut on my own, because I had to admire, examine, and/or fondle almost every slab. Whoa, a pegmatite! Oooh, is that amphibole? Breccia, niiiiice! Ahhhh, my sweet marble (presses cheek up to slab). Oh hey, look at the fossils! Yep, I’m a nonstop stream of exclamation points when it comes to rocks.

But of all those delectable choices, it was sandstone that stole my heart. Wild Sea. A grey to pale green color with graceful, swooping movement, showing how the water flowed just so, a few million years ago. I imagined gentle waves lapping across my kitchen, and of course, I could hardly wait to tell every houseguest about cross bedding and paleocurrents.

Sandstone sometimes gets a bad rap when it comes to its usefulness and durability, but all sandstones are not equal. Some sandstones are porous and not suited for indoor use, but they make excellent dimension stone and are a natural fit for landscaping projects. Other sandstones are dense and impermeable, making terrific countertops and backsplashes. Read on to learn about the all-important differences in sandstones.

Sandstone is made of… wait for it… sand

Presumably, you’ve wiggled your toes on a sandy beach or river bottom. So you’ve experienced the precursor to sandstone: sand. To a geologist, sand is any piece of mineral or rock that is within a certain size range. Sand is between 1/16th of a millimeter to 2 millimeters. That’s somewhere between fine sugar and coarse cornmeal. Particles larger than sand are called gravel or pebbles, and specks smaller than sand are officially called silt.

Sandstone, by the most basic definition, is any stone that’s made mostly of sand-sized pieces that have been stuck together into a solid rock. That loose definition leaves a lot of room for variation. What minerals are in the sand? How tightly is it stuck together? This is why you can’t lump all sandstones together.

Colors tell us where the stone was formed

For starters, it’s helpful to know what kind of sand we’re talking about. Far and away the most common type of sand is quartz, and hence the most common sandstone is quartz sandstone. Quartz sandstone can be white, light grey, red, yellow, or tan. It’s almost always light colored because quartz is clear or white. The colors come from the mineral cement that binds the quartz grains together.

If a sandstone is dark grey or dark brown, it’s probably made of something other than quartz. Bluestone is a well-known example of sandstone that’s made of feldspar, quartz, mica, clay, and rock fragments. Geologists call bluestone a lithic sandstone or “greywacke.”

Dark brown to dark red sandstones and brownstones are a mix of quartz and feldspar and have iron oxide cement. Red sandstones are significant in geology because they formed at times when the atmosphere had abundant oxygen. These “red beds” are also an indicator that the rock formed on land or in very shallow water. Sediments in deeper water are not exposed to as much oxygen, so they tend to be dark grey or black.

Patterns in sandstone

The thing that attracted me to Wild Sea was the pattern made as sand grains were carried along by currents. This is called cross bedding and it’s one of the hallmarks of sandstone. Cross beds look like graceful, diagonal lines that have a gentle curve to them. You know how water flowing over sand leaves a rippled pattern? If you were to slice down into those ripples, you’d see cross beds. The swooping layers show the direction the water flowed.

Small cross beds are made by small stream channels. Giant cross beds are formed by sand dunes. Cross beds that flow one direction, then the opposite direction are caused by water that alternates flow direction—in other words, tides. Cross beds that look like lots of U-shaped patterns show that the water was flowing toward you rather than side to side.

Geologic minutia like cross bedding may seem superfluous. Ah, but no. Cross-bedding is a quick and reliable way to recognize sandstone. This is especially handy because sandstones are sometimes labeled as quartzite or granite. Being able to recognize this distinctive pattern will help you understand what type of stone you’re dealing with.

Some sandstones, like Rainbow Teakwood, have a woodgrain look that is similar to cross bedding. These patterns are caused by dissolved minerals carried in groundwater and they’re called Liesegang rings. You know how a red wine stain on your mother-in-law’s best white tablecloth leaves a distinct, red outline at the farthest reaches of the stain? The same thing happens with minerals too, only with a lot less guilt.

Sandstone evokes ancient landscapes

Think of all the places you encounter sand—on a beach, in a river, or across a desert. These are the same types of environments that brought us sandstones.

The thing we love about sedimentary rocks is how they offer a glimpse into former worlds. Even a billion years ago, beaches and sand dunes formed the same way they do today. We can use what’s happening around us now to infer what happened long ago. That’s one of the fundamental rules of geology.

Sand can be deposited in vast blankets, such as in the Sahara desert. The Navajo Sandstone that spans Utah, Arizona, and Colorado tell us this was a Sahara-like desert during the Cretaceous Period. Massive cross beds, like those seen on sandstone cliffs in Zion National Park, are the remnants of huge sand dunes that marched across the desert floor.

Rivers carry sand, too, with bluestone coming from sandy river channels and river deltas. On the banks of rivers, floods deposit alternating layers of sand and mud. As floodwaters recede, the sediment dries out, shrinks, and cracks into polygon shapes. That’s what made Stone Wood and Palomino. The sand developed a network of cracks, and then dark brown clay filled in the cracks and created a stark geometric pattern.

Beach sand tends to be more pure that river sand, because wave action winnows out everything but the quartz grains. White sandstones like White Sea are most likely from former coastlines. Today, pure white sand can be admired on the beaches near Pensacola, Florida. Someday, those beaches will make a gorgeous sandstone (maybe for my next kitchen?).

How does sand turn into solid rock?

Loose sand can become a super strong stone, or one that’s disappointingly crumbly. It’s all a matter of how well those sand grains are stuck together. Groundwater circulates between sand grains, carrying dissolved minerals along for the ride. Over time, minerals fill in the spaces between sand grains and glue everything together. Geologists call this mineral glue ‘cement.’ The cement can be iron oxide, silica, clay, or calcite. More importantly, the cement can fill in all the spaces between the sand grains, or only some of them. When a lot of those spaces are left open, you end up with a porous sandstone. Some, like Rainbow Teakwood, suck up water like a sponge. Others, like Wild Sea, have very little pore space because minerals have thoroughly filled the spaces between the sand grains, making a low-porosity stone that will repel water. In general, the older the sandstone and the more deeply it was buried, the less porous it will be.

Evaluating sandstones

Given this big disparity in sandstones, it’s important to match the right stone with the right use. For interior use, like countertops, bathrooms, backsplashes, or flooring, you want a low-porosity sandstone. Fortunately, testing the porosity couldn’t be simpler: pour or spray some water on an unsealed sample of the stone and watch what happens. If the water readily soaks into the stone, it’s not a good candidate for use where the stone will get wet. If water doesn’t visibly soak into the stone but it leaves a dark spot, then the stone has moderate porosity, and would be safe for most uses as long as sealer is applied regularly. And if the water neither soaks in nor leaves a dark area, you’ve got a tightly-cemented sandstone that will resist whatever your 4-year old can throw at it.

Quartzite is a former sandstone

Quartzite is all the rage right now. An unfortunate side effect of this wave of popularity is that some stones are being labeled as quartzite when they aren’t. This is most common with marbles that are labeled quartzite, but some sandstones are being called quartzite, too.

Quartzites start out as sandstone, and then they’re subjected to so much heat and pressure that the sand grains fuse together into one solid mass. Even if you look really close, you won’t see individual sand grains. This makes quartzite less porous than sandstone. That said, this process is not definitive. Some stones straddle the line between sandstone and quartzite. Usually this is recognizable by the patterns in the stone. If you can see cross beds, it’s either a sandstone or a quartzite that has only been subjected to a mild amount of heat and pressure. White Macaubus, Nuage, and Infinity White quartzites are examples of lightly metamorphosed quartzites, while White Sea ‘quartzite’ is actually a sandstone. So, regardless of the label, if a stone has cross bedding, check the porosity before falling deeply in love with it.

Many iconic buildings are made of sandstone

Sandstone makes wonderful dimension stone, because it’s hard and impervious to the elements, yet it’s still relatively easy to cut, shape, and carve. That said, quartz-based sandstones and quartzites may require some additional fabrication costs due to additional time or tool wear, since the minerals are so hard. Sandstone buildings are commonplace all over the world, from Manhattan’s brownstones to Washington DC’s Smithsonian Institution to Jordan’s ancient Petra. The US is home to many productive sandstone quarries, such as the Potsdam sandstone in upstate New York, the Dakota sandstone in Colorado, and the Berea sandstone in Ohio.

The University of Wyoming in Laramie takes sandstone construction to a whole new level. They have their own sandstone quarry near campus, with a ready supply of dimension stone. Most of the campus buildings are made of this stone, tying together the campus and lending a native feel to the architecture.

Sandstone is versatile for landscaping

Sandstone has a special place in outdoor spaces. It’s easy to work with and it naturally cleaves into flat slabs for patios, stepping stones, and stone walls. The grainy texture offers grip in wet weather, and it’s manageable to work with as a DIYer. Local sandstones are readily available in most places, and each region has its own sandstone vernacular. In Arizona, red sandstone slabs are a proper fit. Sienna Buff, Sienna Grey, and Moss Rock all hail from southern Colorado. In the northeast, bluestone is right at home.

Here in Montana, I’ve become somewhat addicted to working with our native Frontier sandstone. Most years I buy a couple tons of it and have way too much fun building walls, borders, and pillars. My husband jokes that soon our entire property will be covered in sandstone. Like that’s a bad thing?

 

More from the Geology Series

Bluestone. It’s blue. It’s a stone. End of story, right? Oh no, dear reader, you won’t get off the hook that easily. There’s a lot more to bluestone than its refined good looks. Did you know that bluestone only comes from one region? And that it’s the remnants of a mountain range that doesn’t even exist anymore? And that it can be all kinds of colors? If there’s one thing that’s always true with natural stone, it’s that there’s more to these rocks than meets the eye. Read on to learn why bluestone is a unique American stone that’s as useful as it is beautiful.

 

Bluestone is not just blue

Bluestone is a fine-grained sandstone from Pennsylvania and New York, characterized by its grey-blue color—but it’s not always blue. “There are so many color variations,” explained Bill Mirch, Vice President of Tompkins Bluestone. “Light blue, grey, green, brown, lilac…” Mirch ticked down the list of colors expressed in bluestone. “There are a lot of nice choices.”

Ancient rivers gave birth to bluestone, and this sandstone is the result of a region-wide river system. As with all sedimentary rocks, the particulars of the stone reveal details about its formation. For example, the different colors tell us if the stone was exposed to oxygen during or after its formation. Orange, red, or brown colors are caused by an oxygen-rich environment. Green, turquoise, or blue tones are a result of oxygen-poor conditions, which can occur when decaying organic matter in the sediment uses up all of the available oxygen.

A variety of different ingredients make up bluestone: feldspar, quartz, mica, clays, and rock fragments. In geologic terms, this stone is called a greywacke (pronounced “gray whacky”), which is a sandstone made of a mixture of different particles. Furthermore, ‘greywacke’ is yet another example of how geology is rich with unusual/ridiculous vocabulary terms!

But this jumble of ingredients tells us something about bluestone. As sediments are transported farther from their source, they sort themselves out into similar minerals of similar sizes. But bluestone, being made of a diverse mix of ingredients, is made up of sediment that travelled a relatively short distance down a river. It also hints at the fact that all bluestones are not identical. The range in colors, layering, and texture is one of bluestone’s best assets.

 

Bluestones are the remnants of an ancient mountain range

Let’s do a little time travel back to the Devonian Period, nearly 400 million years ago. A mountain range, called the Acadian Mountains, was being uplifted along the east coast of North America. As a tectonic collision cranked the mountains upward, erosion sought to wear them back down. Rivers carved out valleys and carried away the sediment. At the foothills of the mountains, the rivers met an inland sea, the currents slowed, and the rivers laid down blankets of sand, gravel, and clay. More sediment piled on top of that, and the pressure from the overlying layers helped glue the particles together to create solid rock.

A huge range of rocks got deposited in this manner, and all together they are called the Catskill Delta Formation. This deposit is up to 10,000 feet thick, and covers parts of eastern Pennsylvania and southern New York. The Catskill Formation includes black shales, thick beds of coarse red sandstone, occasional limestones, and lenses of bluestone. The bluestones were formed from sandy deposits in river channels and river banks. Layers of shale and siltstone are usually found above and below the bluestone layers.

Bluestone was not laid down in a continuous blanket. Instead, rivers and deltas left behind pockets of sand. As rivers shifted across the landscape, the sandy layers did too. As a result, bluestones are found in small, discontinuous layers scattered throughout the region.

It’s amazing to think that the Acadian Mountains that shed their sediments to create the Catskill Formation no longer exist at all. They’ve been erased by erosion and overridden by the Appalachian Mountains, which formed in a similar way, about 50 million years after the Catskill Delta. The Appalachians, of course, are still with us today, but at some point they’ll lose the battle with erosion too, and will be worn away by restless rivers.

 

Geologic variability leads to a versatile stone

The geologic origins of bluestone conspired to create a particularly useful material. The sand grains of bluestone have been tightly cemented together, yielding a strong and durable stone. It has a consistent texture that can be left as-is, or can be worked into a variety of different finishes.

Bluestone’s natural layering allows the stone to be cleaved into agreeable flat shapes, and variations in the thickness of the natural layers provides a range of stone for different uses, from chunky blocks to thin sheets.

The way that bluestone was deposited in individual layers and pockets gave rise to small quarries scattered throughout eastern Pennsylvania and southern New York. The small scale of the operations favored close-knit crews working by hand rather than with heavy machinery. “Information was handed down from generation to generation,” explained Mirch. “It was a family thing.”

Because bluestone layers fade in and out across the landscape, quarrying it “is really an art,” said Mirch. In some places the sandstone beds are thick, which are useful for stair treads, curbing, and sills. Thinner layers are well-suited for paving or patio stone. Broad slabs are ideal for sidewalks. Because of the variability, “it’s harder to understand how to extract it,” offered John Malyshko, owner of Natural Stone Resources, Inc., but the upside is that, “different products come from different parts of the formation.”

 

“The precursor to concrete”

Bluestone has been a useful stone since the 1800s, when it began to be quarried for curbs, sidewalks, door sills, window sills, and paving stones. Much of the stone was shipped to nearby New York City, where “it was the precursor to the concrete sidewalk,” said Malyshko. “You can still see 200 year old bluestone steps and paving material.”

A spectacular example of early bluestone construction is the Starrucca Viaduct, a 1,000-foot long, stone railroad bridge in Lanesboro, PA. The viaduct was built of Ashlar bluestone in 1848, and was completed in only a year, thanks in part to the fact that the quarry was just 3 miles from the viaduct. If you had any doubts about the strength and durability of bluestone, take a look at that structure and appreciate that it’s still being used for commercial rail freight today.

 

Many uses for bluestone

Today, bluestone remains a popular choice and is one of the most abundant American natural stones. Patios, stair treads, stone walls, and pool coping are common uses of bluestone. In part, bluestone remains in demand because of its versatility. Daniel Wood, natural stone consultant for Lurvey Supply and chair of the Natural Stone Institute’s education committee, described the many facets of bluestone. “You can work it and craft it. You can have different sizes, different surface textures. It can be layered or not.”

The natural layers of bluestone can be “cleft,” or broken apart, yielding a naturally flat surface with just a hint of texture. In the early days of bluestone, its tendency to break along flat layers was part of its appeal.

But now, Mirch explained, diamond-blade saws have transformed the industry and ushered in myriad new uses for bluestone. No longer reliant on natural layers, stone cutters can saw the stone into specific sizes and thicknesses for architectural use, and mill it into thin tiles. “We’ve expanded our market to the [home’s] interior,” said Mirch.

Different finishes can mesh with different parts of a home. Bluestone can be tumbled to give it an antique look, it can be brushed or honed for smooth floor tile, or a ‘flamed’ finish can be applied, which leaves a planar surface with a slightly rough texture. Mirch summed up bluestone’s adaptability: “It goes from rustic to contemporary, very easily.”

Using the same stone in different ways can unite different parts of a project. “There’s a rhythm; a simplicity of materials,” said Wood. “It ties together into a cohesive design.”

Even bluestone’s leftovers are useful. Thinner layers can be made into tile, and crushed bluestone can be used as gravel. “There’s so many things you can do with it, or to it,” remarked Wood.

 

‘The industry is thriving’

Even though the bluestone industry is made up of small and medium-sized quarries, the stone has a huge reach. Demand for bluestone is highest in the northeast. “It fits the vernacular of the region,” Wood said.

But bluestone is shipped far and wide. “It’s very popular, nationwide,” explained Malyshko. “It’s like bread or butter. People know the material. It’s a safe choice.”

“The industry is thriving. The quarries are doing great,” observed Wood. “They can’t make it fast enough.”

Malyshko echoed this sentiment: “Every block that’s extracted is sold.” Mirch added, “We never have a surplus. Ever.”

For example, Tompkins bluestone supplies 6,000 to 8,000 square feet of bluestone sidewalks for historical neighborhoods in New York City every year. Mirch appreciates the continued use of traditional natural stone. “I really can’t get jazzed over a concrete sidewalk,” he quipped.

Furthermore, this American stone fosters American jobs, and not just in the quarry: “Trucking, machining, diamonds, blades…” Malyshko counted off the ways that bluestone stokes the local economy.

Perhaps bluestone is the quintessential American success story. A geologic remnant became a useful product, and small-scale quarrying fostered a bustling industry. Over time, quarriers, masons, and architects innovated new ways to use this old stone. And all the while, customers have flocked to it, keeping the industry vibrant and allowing the cycle to continue. Bluestone’s easygoing versatility shows us that sometimes the simplest ideas are the most durable.

Photos courtesy of Natural Stone Resources, Inc.

More from the Geology Series

From the shimmering aura of the Taj Mahal to the humble floor of your home’s bathroom, marble is one of the world’s most revered and useful natural materials. Marble is Michelangelo’s David, the Washington Monument, and the Duomo of Florence. It is also the primary ingredient in Tums antacid.

Tune into any kitchen design message board and you’ll encounter a sharp divide over the utility of marble. Some people wouldn’t dream of designing a kitchen without marble. Others decry that choice, citing marble’s imperfections and fragility. Many are caught somewhere in the middle: in love with marble’s irresistible appeal, but uncertain if it’s the right choice for their circumstance. In an attempt to create the “perfect” material, legions of synthetic products claim to look “just like marble,” but marble enthusiasts aren’t swayed. Few stones can match the warm glow, the softly flowing colors, the timelessness, and the authenticity of marble. There’s good reason this stone has been used for over 5,000 years.

A seabed, transformed

Marble is a metamorphic rock; it once was a different kind of rock, and was then transformed by a change of circumstance. Before marble becomes marble, it is first limestone, which forms on the shores and floors of tropical seas. Limestone is an accumulation of shells, shelly fragments, microscopically tiny shells, and dissolved shells. Depending on the conditions at a particular beach or sea, limestone’s shelly sediment may be punctuated with occasional layers of clay or lenses of sand. But by and large, limestone is an assortment of shell-remnants, which are made of the mineral calcite.

The tropical shoals that give rise to limestone do not stay peaceful indefinitely. Oceans are basically geologic conveyor belts. The ocean floor slowly spreads apart from the center and slips underneath continents at the edges, gradually rearranging the map of the world.

When a limestone seabed gets dragged down into the Earth’s crust, the additional heat transforms the calcite grains and fuses them together tightly. The dynamic action of rock layers as they become buried, twisted, and shoved around causes the original flat-lying layers to bend, buckle, and swirl together. A rock in this heated state doesn’t melt. It’s simply warm and flexible, much like a chocolate bar left in your pocket. This process of heating and warping is responsible for marble’s trademark aesthetic of gracefully flowing bands of color. The grey swirls in marble are simply clay layers that got folded, smeared, and re-folded into the marble like a ribbon of chocolate infused throughout fudge ripple ice cream.

Patterns and colorways give marble versatility and character

Part of what makes natural stone so appealing is the huge range of colors and variations expressed in solid rock. On one hand, these patterns tug on our heartstrings and offer aesthetic possibilities. On the other hand, each of these variations has its root in some sort of geologic process.

Most marble is white. The classic, white marble is pure calcite, without intervening minerals to lend it color. Yule and Thassos marbles are well-known examples of pristine white marbles. [Read more about white stones here.]Marble can take on a delightful range of hues and textures. Pink marble is tinted by iron oxide, as is golden marble. Green marble and deep red marble contain serpentine, rich in magnesium. Fossil-rich limestone becomes graphite-infused marble, as the carbon in fish, algae, or seaweed reverts back to the elemental carbon in graphite when the rock is heated. Graphite-rich marble is steely grey with a metallic glimmer.

A marble breccia is formed when the stone fractures underground, which can happen if a stone lives in a fault zone. Mineral-laden groundwater comes to the rescue, filling in the voids and patching the fragments back into a solid mass, while creating a spectacular pattern at the same time. Arabescato is a beautiful example of a marble breccia.

It’s important to understand the properties of marble

The mineral content of marble is the same as the limestone it came from, and both of these stones are made of calcite, AKA calcium carbonate, AKA CaCO₃. Calcite is one of the more common minerals on Earth’s surface; in addition to limestone and marble, calcite is the primary ingredient in travertine and onyx.

Calcite has a few properties that you should know about before you fall head over heels in love with a stunning marble slab. Calcite is 3 on Mohs hardness scale, which means it will get scratched by knife blades, ceramics, and a cast iron skillet accidentally slid across the kitchen island.

Calcite is also chemically reactive with common acids, such as those found in lemonade, wine, and colas. When acidic liquids land on a marble slab, a tiny amount of the stone is dissolved, or “etched.” This doesn’t affect the integrity of the stone, but it does leave a slight change in the color and/or luster of the stone. On a polished stone, an etch looks like an unpolished area.

Etches can be polished out, or they can be left alone and considered part of the natural patina that marble will acquire over time.

Marble is ground up into antacid tablets because calcite neutralizes acid, which makes your stomach feel better. That also explains why acid makes a mark on marble. The marble reacts with the acid, neutralizing the acid, but damaging your countertop in the meantime. Ironically, when your teenager dribbles pickle juice on a brand new countertop, you may find yourself reaching for the antacid, triggering the same chemical reaction both on the countertop and in your digestive tract.

The last piece of potentially bad news is that marble can form “star cracks” or “stun marks” if a heavy, hard object falls on it. Star cracks look like small, light-colored areas where the impact occurred. This is usually an aesthetic issue, not a structural one, but if an impact occurs at the edge of the stone, it can chip or flake.

A common misconception is that marble stains easily. However, this is actually quite rare. The metamorphism that bakes the stone also knits the minerals together tightly. Porosity for marble is similar for that of granite. That said, the porosity of all stones varies, so check the stone specs and do your own tests with a sample of stone. Marbles are typically sealed to reduce the likelihood of staining. Alas, sealing does not make marble any less prone to etching, I’m sorry to say.

Now you can see why some people love marble while others think that marble-lovers are crazy. To some, the nicks and etches on a marble surface add character and mark the passage of time in a busy household. To others, each blemish resonates as a personal loss. Which way do you see it? This is an important question to consider before you head to the slab yard. [Check your stone personality to see what stone suits your temperament.]

Marble with magnesium = Dolomite

Dolomitic marble is a close relative of regular marble. Standard marble is made of calcite (CaCO₃), and dolomitic marble has a little magnesium in the mix (CaMgCO₃). There is not a huge difference between the two variations, except that dolomitic marble is a little bit slower to etch. You’ll have a moment to wipe up a spill before the chemical reaction takes place.

Marble with quartz = Confusing

While marble is primarily made of calcite, it’s possible for the original limestone to have occasional layers of sand or chert (chert is a marine rock made of pure silica). These interlopers turn into areas of quartz as marble undergoes metamorphism. The end result is a stone that is mostly calcite with some quartz. This combination of ingredients has kicked off industry-wide confusion, because calcite and quartz have distinctly different properties but they look alike.

Unfortunately, marble that contains minor amounts of quartz is sometimes labeled “soft quartzite,” which is both an oxymoron and a misnomer.  There is no such thing as soft quartzite, and that term should be avoided by dealers, designers, and customers alike. Marble that contains small areas of quartz is still marble and should just be called marble. Super White is one well-known example of a mislabeled stone. Super White is a dolomitic marble with occasional bits of quartz. Super White is neither quartzite nor “soft quartzite.” It’s marble, and a gorgeous one at that. [Learn more about the quartzite/marble labeling problem in The Definitive Guide to Quartzite.]

 Limestone is sometimes classified as marble

The term “marble” is often applied broadly rather than literally. Many stone restoration professionals categorize marbles, limestones, and travertines together as a family of stone because they require similar maintenance and refinishing techniques. Polished limestone is sometimes called marble. While there’s not a huge difference between the two, marble is much denser and therefore is resistant to staining. If a slab has fossils, shell fragments, or has open pockets within the stone, it is limestone.

Many stones classified as black marble are actually black limestones. This is particularly true for dark colored stones with stark white veins, like China Black, Dynasty Brown, or Nero Portoro. Because marble has origins as a fluid, heated rock, its stripes and veins are usually soft, flowing, and curved rather than angular.

Many ways to bring marble into our lives

Marble remains a popular choice for countertops, backsplashes, bathrooms, tabletops, flooring, and cladding. Marble’s versatility makes it at home in an ancient Greek sculpture, in a lavish hotel lobby, or on a hardworking kitchen island. Marble also finds its way into our lives as household objects like cheese boards, rolling pins, vases, and lamps.

Despite the emergence of marble lookalikes, there’s nothing quite like the real thing. Real marble has qualities that cannot be replicated in a lab. I recently visited the 9/11 Memorial, a sobering space of monument, museum, and reflection. Next to the footprint of the fallen towers, rises the one part of the site that inspires optimism. Called the Oculus, it is part transit center, part shopping mall, and its soaring white ribs beckon investigation.

Visitors who step inside are rewarded with a vast, cathedral-like space, covered in pure white marble. The combination of natural light, natural stone, and creative architecture transform the somber mood into a hopeful one. Wandering around the expansive structure, I finally put my finger on my favorite quality of real marble. Light penetrates into the white stone, then radiates back out in heavenly luminosity, filling the room with a warm, soft glow. Leave it to a stone like marble to completely alter the mood of a building. Marble has been a metaphor for worship, reverence, and beauty for millennia. Its ability to do that is all the more appreciated today.

More from the Geology Series

“That is not granite.” I said firmly, brimming with 24-year old certitude. The salesperson gave me a look over his glasses, annoyed. In his hands was a sample of Black Pearl.

“It’s anorthosite,” I said, taking the stone, turning it in the light, and admiring the reflections off the steel grey feldspar crystals. “There’s no such thing as black granite.”

The salesperson softened and we had a lively conversation about stones from Norway, upstate New York, and other localities. While our approaches to stone came from different directions, we shared an absolute love for them.

Scenes like this probably unfold every time a geologist walks into a stone showroom.

To a geologist, granite is one specific type of stone. It’s a light colored, coarse-grained igneous rock. Thus, “black granite,” to a geologist, is an oxymoron akin to a “vegetarian steak.” There simply is no such thing.

But neither side needs to be right or wrong on this. Naming conventions for stones vary across different industries. Those who work in mining, gemstones, commercial stones, and geology all have their own language. And that’s okay. Everyone should use the names that make the most sense for their purposes. The stone industry needn’t start labeling slabs as granodiorite or monzonite just to make geologists happy.

What’s in a name?

In the natural stone industry, “granite” often means any hard stone that is not marble. Different dealers use the term granite slightly differently. For example, quartzite and sandstone are sometimes classified as granite. Ideally, the names of stone reflect what they actually are. The more precise the definition, the clearer it is for everyone: consumers, salespeople, architects, designers, and fabricators. That said, stones with the same traits can be lumped together for simplicity’s sake.

Perhaps the most useful definition for commercial granite is one that includes all igneous rocks, plus gneiss and schist. This grouping makes sense because these stones have similar properties, are generally made of the same minerals, and can all be used in similar ways.

This article will talk about igneous rocks that comprise most of the granite family. Gneiss is a metamorphic rock that has been heated and compressed to the point that a linear pattern develops, giving the stone flowing stripes of color. Schist is another metamorphic rock that contains abundant mica, which imparts a glittery look to the stone.

Defining granite

Let’s talk about true, geologic granite for a moment.

Geologist’s granite is made of four minerals: feldspars, quartz, mica, and hornblende. The feldspars can be white, light grey, and/or pink. Quartz is usually glassy grey, the mica is black or silver flakes, and hornblende looks like black rectangles. Each of these ingredients is large enough to see and point to. The stone has a light color and a chunky texture that contains all of the colors evenly distributed throughout the stone, without any particular pattern or grain. Examples of geologist’s granite are Barcelona, Giallo Atlantico, White Mount Airy, and Luna Pearl.

Granite is but one of many types of igneous rocks. Like most scientists, geologist love to organize and classify things, and there are all kinds of important meanings behind the mineral contents and textures of igneous rocks. But for commercial use, the minerals and colors are mostly a matter of aesthetics. Thanks to the variations of melted rock, we can choose from igneous rocks that are pale grey, creamy tan, warm bronze, dark green, or stark black.

Granite forms in magma chambers

Granite and all the other commercially useful igneous rocks form in a similar way. They all start out as molten rock that cools underground. Unlike lava that erupts from a volcano with dramatic flair, granite does not erupt. It stays underground and slowly cools down, changing from a thick liquid to a solid mass of crystals. Depending on the depth and size of the magma chamber, this process can take thousands to millions of years.

In general, the longer it takes for a stone to turn from fluid magma to solid rock, the larger the crystals. If the whole magma chamber cools down at the same rate, the rock will have an even texture where all the minerals are the same size (like Barre Grey). If it cools at different rates, you’ll get minerals of varying sizes (like Baltic Brown). Fine-grained stones like Absolute Black cooled relatively quickly and did so near the Earth’s surface where it’s not that hot. Coarse-grained stones like Uba Tuba cooled more slowly.

In geology, what happens underground does not stay underground. Subterranean rock formations are heaved upward by tectonic stresses. Overlying layers are stripped away by erosion. Alas, we can stare right at the solidified contents of a magma chamber.  The Sierra Nevada Mountains are a classic example of granite that cooled deep underground, was tilted upward by faults, and then was sculpted into soaring cliffs and pinnacles by ice age glaciers.

Variations in granite… and similarities among granite

Within a given quarry there are often variations of colors and textures. The edges of a magma chamber can capture rocks from the walls of the chamber, introducing xenoliths (meaning “foreign rocks”) into the mix. Or, sometimes pockets of magma will shoot off into the chamber walls, creating dramatic veins of color. Some of the golden-hued granites are colored by groundwater that oxidizes the iron-rich minerals in the stone. All of these processes help explain why a single quarry can produce several variations of the same stone.

Even though igneous rocks span different colors, textures, and mineral types, their overall properties are consistent. All of these stones are hard enough so they won’t get scratched with normal use. They won’t etch, even when red wine is left pooled on the countertop all night (I have no idea how that could happen). In general these igneous rocks are not very porous and don’t stain, but that can vary with individual types of stone.

Special granites

Pegmatites are granites with huge minerals in them. Alaska White, Delicatus White, Patagonia, and Alpine are examples of pegmatite granites. Pegmatites are often the last part of the magma to solidify, and the presence of leftover water in the magma makes it easy for minerals to grow. And so they grow and grow, until they become a foot or more across! Read more about pegmatites here.

Granulites are white or light golden granites that have a subtle linear grain in the stone, and they also commonly contain dark pink garnet crystals. These stones are a metamorphic version of granite, and are popular because they offer soft colors and patterns that are easy to incorporate into a kitchen or bathroom. Colonial Gold, River White, and St. Cecilia are examples of granulites. Read more about granulite here.

The Pearl family (Blue Pearl, Labrador Pearl, Emerald Pearl, etc) are deservedly popular granites, known for their tenacity in a busy kitchen and their agreeable aesthetic. These stones are characterized by the iridescent silver or blue minerals that can give the stone a bit of shimmer or a hologram-like wow factor. The mineral that causes the shine is labradorite, which is a type of feldspar. Labradorite is an uncommon color variation of a very common mineral.

Uses of granite

There’s a reason why granite is so well known and widely used. It’s basically bulletproof. Walk around any city in the US, and you are likely to see granite used in building stone, monuments, landscaping, and even curbing. Step inside many kitchens and baths, and you’ll see even more granite. Granite is a trouble-free and easygoing choice, with enough variations in color and pattern to compliment almost any aesthetic.

As with most stones, there are a few caveats when designing with and using this durable stone.

Some light-colored granites and granulites can be porous, which means they can stain. A basic test for porosity is to put a splash of water on the stone. If the water beads up and remains in a bead, the stone does not need sealing. If the water eventually soaks in and leaves a darker area, the stone needs to be sealed.

Black granites have the opposite problem. They are so dense that sealers can create a cloudy finish. Moreover, some dark granites can become spotted by the minerals in tap water. As with any purchase of any natural stone, learn as much as you can about the stones you’re considering. Ask for samples to bring home and test for staining and overall durability. Work with reputable fabricators that have experience with the particular stone you’re planning on using.

Despite its slightly confusing name, it’s worth getting to know the many shades of granite. As Juliet said to Romeo, “a rose by any other name would smell as sweet.” We know that granite, by whatever name you like to call it, is just as awesome.

More from the Geology Series