Know Your Minerals

by Karin Kirk | Aug 30, 2019 | geology granite limestone

Our thanks to Slabworks of Montana and Montana Tile and Stone for allowing us to explore and photograph their beautiful stone slabs.

A handy guide to the ingredients of your favorite natural stone

Minerals are the components of all natural stones. The color of every natural stone, whether it’s jet black, glittery silver, or a kaleidoscope of Technicolor – comes from the individual minerals.

Minerals also give each stone its personality. Is it flashy, or subtle? Is it a uniform color, or a melee of diverse ingredients?

And, of course, minerals dictate the properties of a stone: Hard or soft; acid-resistant or acid-sensitive; flaky, chunky, or smooth.

Given that minerals determine so much about a stone, it warrants a look at some of the more common minerals, how to spot them, and what they tell you about a stone.

Before we dive in, one important point is that you can only see individual minerals in coarse-grained stones. A smooth stone with small grains, like Absolute Black or Pietra Grey, doesn’t reveal much about specific minerals because you can’t see them. But many popular stones have big crystals in all kinds of patterns and colors, inviting curiosity about just what those minerals are all about.

Feldspar

Feldspar is the most abundant mineral in the Earth’s crust, but it’s far from mundane because it occurs in a huge range of colors and forms. When you look at a slab of typical granite, you’re looking at mostly feldspar. Igneous rocks like granite take shape as they solidify from liquid magma. You can think of magma as a ‘slushy’ drink. It’s a mixture of solid bits and liquid. Feldspar crystals are often the solid chunks within a body of slushy magma, and you can see the chunky texture of feldspar in some granite slabs.

In other cases, feldspar is altered by metamorphism, changing its shape from pushing, pulling, or shearing. In these cases, the blocky shapes of feldspar crystals can become more rounded.

Color:

Feldspar crystals can be white, black, and any shade of grey. They can also be pink, cream, brown, and sometimes green. Best yet, the variety of feldspar called labradorite is iridescent blue, and plays the starring role in Blue Pearl and Volga Blue.

Identifying features:

Since feldspar can be nearly any color, using color to identify it won’t help at all. But it does have a few features that make it recognizable.

Feldspar is not glossy and its luster is similar to porcelain when it’s not polished.
It is always opaque, meaning, you can’t see ‘into’ the crystal at all.
It’s more or less rectangular in shape.
The crystals break into naturally flat faces called cleavage planes. This is especially visible on a honed or leathered slab when you look at it from an angle. The flat faces will catch the light. This is also evident on the edge of a slab where you can see a crystal in three dimensions.
You can sometimes see subtle stripes or grooves in a feldspar crystal.

Properties:

Feldspar is an all-around easygoing mineral. It has no special needs, has a Mohs hardness of 6 to 6.5 (harder than glass), and holds up well to hard use. It won’t etch, flake, or offend anyone in any way.

Examples: Alpine White, Antique Brown, Patagonia, Coral Grey, Blue Pearl, and many others. Since feldspar is such a common mineral, there are dozens of examples.

Superpower: Feldspar makes you more gracious and less self-involved. The relaxed flow of energy from feldspar allows you to let someone else take that last slice of pizza, even though you’re still a little bit hungry.

: Feldspar often forms rectangular or blocky-shaped crystals. In this slab of Ocre Itabira there are two types of feldspar. One is silvery white and the other is light brown.

: In this slab of Sea Foam Green it’s plain to see that the feldspar crystals floated around in a liquid magma.

: These feldspar grains lost their blocky shape when the stone got squeezed and stretched during metamorphism. The black, wavy layers are black mica.

: All feldspar, all the time! This stone is 100% feldspar, arranged in a textbook-perfect example of ‘interlocking’ texture that shows how the crystals have grown tightly into each other. The different shades of grey are caused by the different orientations of the crystals, so they reflect light in different directions. This is leathered Antique Brown, which a geologist would call ‘anorthosite,’ and I would call ‘fabulous.’

: Blue feldspar – called labradorite – in Volga Blue.

: Dark grey feldspar showing striking parallel lines called ‘twinning’ in a leathered slab of Antique Brown. If I could have snuck this slab into my car and taken it home, I would have.

: Often the feldspar crystals are larger than the other types of minerals. As the stone solidified from magma, the feldspar solidified first and had more time to form large crystals. Those large crystals are called phenocrysts.

Quartz

Quartz is the second most abundant mineral on the Earth’s crust. Even though it occurs in similar colors as feldspar, it has a distinctly different look.

Colors: Clear, white, grey, black, pink, amber, brown, purple. Some colors of quartz have their own name. Purple quartz is amethyst. Amber quartz is citrine. Grey quartz is smoky quartz, and so on.

Identifying features:

Once again, color is no help in identifying quartz, but quartz is easy to spot once you see it a few times.

Quartz is often translucent, meaning you can see down into the crystal.
Quartz looks a lot like colored glass, because it has the same chemical composition as glass.
Unlike feldspar, quartz does not have a ‘preferred’ shape and it almost never breaks along a flat plane.
When looking at an igneous rock, quartz is often blob-shaped, because it’s the last mineral to solidify and it fills in the gaps left open between the other minerals.
In a metamorphic rock like gneiss, schist, or a quartzite like Fusion or Marine Blue, quartz forms white blobs that are often squiggly-looking. In these cases, the quartz melted as the rock was undergoing metamorphism.

Properties:

Quartz is heralded for being beautiful, durable, inert, and generally bombproof. It’s 7 on Mohs hardness scale and isn’t bothered by acids or less-than-ideal cleaning habits. In fact, quartz is such a cool mineral, that a type of manmade countertop material borrowed its name, leading to endless confusion within the countertop industry.

Examples: Quartz is prevalent in light colored granites like Alaska White or Bianco Antico. And, of course, quartz plays the starring role in quartzites like Taj Mahal, Fusion, and Sea Pearl. Quartz is also the main ingredient in Wild Sea sandstone.

Superpower: Having quartz-rich stones in your home makes you less likely to snack between meals. No one knows why.

[Disclaimer – these superpowers are not to be taken literally. Hopefully you already realized that?]

: Quartz looks like glass. You can see ‘into’ it. It also has an irregular shape.

: Areas of white quartz in a slab of Titanium. In this case the quartz melted during metamorphism, forming fluid blobs within the rest of the stone. Geologists would call this stone a ‘migmatite,’ meaning ‘mixed rock,’ because it’s part metamorphic, part igneous.

: This slab of Allure is 100%, crystalline quartz. Notice that the crystals don’t have any particular shape.

: Geologists call this ‘graphic granite,’ because the quartz looks like ancient cuneiform writing. It’s an intergrowth of feldspar (the white background mineral) and quartz (the grey bits). Once you recognize this pattern, you’ll notice it on a lot of slabs. This is Alaska White.

: A more subtle example of graphic granite. The taupe-white background mineral is feldspar, and the thin, glassy grey slivers within it are quartz. On either side of that is a nice smattering of garnet crystals. Lastly, the black slivers are biotite mica.

: Translucent and blob shaped… you might think this is quartz. Alas, it’s epoxy fill. I’ve been fooled by that more than once!

Garnet

Garnet is January’s birthstone, and it’s a beautiful mineral. Garnet is dark-raspberry pink, maroon, or maroonish-brown.

Identifying features:

The color is usually a total giveaway.
The crystal shape is usually round-ish. It often occurs in specks.
Garnet crystals have a glassy luster, and when viewed on the edge of a slab or in an un-cut rock, they are brilliantly sparkly.

Properties:

Garnet is 6.5 to 7.5 on Mohs scale and is often used as an abrasive. Large, translucent crystals of garnet are used for gemstones.

Examples: Garnet is not a major ingredient in any stone, but its recognizable crystals are in River White, Colonial White, Dallas White, and St. Cecilia, among others.

Superpower: People who like garnet are above average at math. Unfortunately, simply owning a garnetiferous stone will not, in and of itself, improve your math skills.

: Garnet’s dark pink color and round crystal shape make it easy to recognize.

: These garnets come to life in the sunshine of the slab yard. The white mineral is quartz and the black is biotite mica.

Mica

Mica is the defining ingredient in sparkling stones like Orion and Magma Gold, and is a minor player in granite and gneiss. Mica is nature’s glitter: it’s shiny and forms in flat flakes. It’s hard to identify mica in a polished surface, but if you look at the edge of the slab you can usually make out the trademark shape.

Identifying features:

When you view mica crystals end-on, you can see the very thin edges of the flakes. Sometimes these are stacked like pages in a book.
Mica sheets are pliable and you can likely bend them with your fingernail.
The mica family includes minerals of silvery white, golden, brown, and black. There is even a green mica called chlorite that gives green slate its color.

Properties:

Mica is best in small doses, because it’s soft and does not conform to polishing like other minerals. Small-grained mica is ideal because you can enjoy the sparkle without having the stone flake apart. Mica is a major ingredient in schist.

Examples: Many granites and gneisses have small amounts of mica minerals. Some examples include Delicatus White, Vahalla, Rocky Mountain, or Viscont White. Schists contain larger amounts of mica, as can be seen in Desert Dream, Galaxy Schist, or Saturnia.

Superpower: Mica makes you more tolerant of your family members.

: Black mica is called muscovite, and it’s seen two ways in this photo. On the edge of the slab, the flat surfaces of the flakes are visible. On the polished surface of the slab in the background, you can see thin slivers of the edges of the mica flakes. This stone is Orion.

: Silver mica is called muscovite. The edge of the slab is the best place to spot the thin layers within the mineral. The black, blocky grains behind the mica are amphibole. The white is feldspar – and you can really see the non-glossy luster of feldspar in this photo.

: When viewed on the edge of the slab, mica’s distinctive thin sheets are easily recognizable. In this case, the layers of this schist are folded into wavy layers, which is a common theme in metamorphic rocks.

: This pretty slab of Persian Pearl is mostly quartz with thin slivers of black mica and a few garnets.

Amphibole

Black flecks in an otherwise light granite are either amphibole or black mica. Amphibole and black mica are also the duo that makes up the dark stripes in gneiss. (The light stripes are feldspar and quartz.) Amphibole is also called hornblende.

Identifying features:

Amphibole is an even, jet black.
It tends to form crystals that are skinny rectangles, but sometimes it also has an irregular, blocky-ish shape.
You can tell the difference between mica and amphibole by shape. Mica = thin flakes, and amphibole = thin rectangles. It’s subtle, but different enough to be a reliable way to tell these minerals apart.
When in doubt, look at the edge of the slab to determine the crystal shape.

Properties:

Amphibole has a hardness between 5 and 6, which is similar to or slightly harder than glass. It takes a polish well, and does not need any special care. In most stones it’s a minor ingredient.

Examples: Amphibole is usually a minor ingredient in slabs, but it makes a spectacular appearance in Crowsfoot Schist, Montana Brown, and Alaska White. Stones like Barcelona, Roca Montana, and Bianco Antico have more subtle crystals of amphibole.

Superpower: Eases headaches; but also may cause cravings for salty things or Chinese food.

: Amphibole showing off its characteristic black rectangles. The white is feldspar, and the copper color is quartz. The small flecks of black just above the left side of the pencil are black mica. This stone is called Sedna.

: Amphibole crystals are the dark brown or black rectangles. The clear, grey, glassy vein is quartz. There are also white feldspar crystals throughout this slab, and some garnets near the bottom of the photo. This is Montana Brown.

: Don’t ask me to explain how this piece of amphibole got this interesting shape. But it sure is cool! The slab is Alaska White.

Calcite

This humble mineral expresses itself in so many wonderful ways. Calcite is the main ingredient in marble, limestone, travertine, and onyx.

Color:
Calcite is almost always white or nearly white, and it can have tones of other colors like cream, apricot, light brown, light green, light grey, or light pink.

Identifying features:

Calcite can look a bit like quartz (hence the perpetual confusion between marble and quartzite), but it has a few differences.
- Calcite has a satin luster, while quartz looks glassy and is more translucent.
- Calcite forms crystals with flat surfaces, and also breaks along flat planes. So when light reflects off calcite, you see glints of light from flat surfaces (see photo).
- When in doubt, go by the properties rather than the looks.
Calcite is easily dissolved in groundwater and it often fills in cracks in rocks. Bright white stripes in dark colored limestone or marble are veins of calcite.
Just to make things more confusing, quartz can also make light colored veins in a stone. A quick hardness test with a pocketknife will reliably tell you which is which.

Properties:

The best way to tell calcite from quartz or feldspar is by its properties. Calcite has a Mohs hardness of 3, which is harder than a fingernail but softer than glass. A sharp-tipped nail or pocketknife will leave a definitive scratch in calcite. Also, vinegar dripped onto calcite will often gently fizz.

These same properties apply to slabs and tiles. Stones made of calcite can be etched from acids, and can also be scratched by metal or ceramic kitchen implements.

Examples: All marbles and limestones are made of calcite. Some well-loved examples are Carrara, Danby, White Cherokee, and Colorado White marbles; or Belgian Blue, Fossil Black, Saint Pierre, or Emperador limestones.

Superpower: Calcite makes it easier to keep world events in perspective and has been shown to reduce social media use.

: Calcite crystals reflect light off of their flat planes. Leathered Fantasy Brown.

: Calcite can look similar to quartz because it’s translucent, but quartz is glassy while calcite has a softer, satiny luster. But a quick hardness test will sort it out definitively. This is a slab of onyx.

: Calcite fills in pore spaces in this limestone. On the edge of this slab is a small, crystal lined pocket.

Dolomite

This is calcite’s cousin, and shares similar color and properties of calcite with a few exceptions. Compared to calcite, dolomite is slightly less susceptible to etching from acids, giving you some time to wipe up spills before the stone is affected.

Identifying features:

Dolomite and calcite look alike and you can’t tell them apart visually.
The test for dolomite is called the ‘powdered rock acid test.’ Calcite will have a fizzing reaction when in contact with diluted hydrochloric acid. Dolomite will not, unless the stone is powdered first. To do this test, use a nail or tip of a knife and scratch up an area of the stone. Leave the stone dust in place, and then put one drop of acid on it. If the stone is dolomite, it will have a weak, bubbling reaction. If it’s some other type of mineral altogether, like feldspar, there will be no reaction at all.

Properties:

Dolomite has a Mohs hardness of 3.5, which is slightly harder than calcite. Dolomite also etches more slowly than calcite, but it will still etch.

Example: Super White a well known dolomitic marble that is frequently mislabeled as quartzite.

Superpower: Proximity to dolomite strengthens teeth and makes dental visits less stressful.

This is not a natural stone – it’s a slab of engineered quartz. One of the reasons manmade quartz does not look natural is because the ‘crystals’ are all blob-shaped and have the same luster. The recognizable rectangles of feldspar and glassiness of real quartz are absent.