Comparing the performance attributes of natural stone with manmade materials

Interior Design

Styles may change from season to season, but Mother Nature creates unique designs in natural stone that are historically innovative and always on trend. This is why natural stone remains a timeless and flexible option for many interior applications including countertops and flooring. The variety of natural stone available is also beautiful, durable, and versatile, offering endless possibilities for interiors.

Manmade materials often attempt to emulate or replicate these characteristics and qualities. It is essential to understand how natural stone compares to some of the manmade materials used for the same applications in order to select the most appropriate material. It is also important to know how the material chosen for an interior project will perform under a variety of circumstances, conditions, and uses. At the same time, knowing how the material impacts the environment will ensure that you are choosing the most sustainable material for the application.

Coastal Sand limestone. Photo courtesy of MSI.

Performance Matters

A material’s optimal performance is based on a combination of the attributes of the material itself, how the material is installed, and the conditions to which it is exposed. There are many different issues to consider for interior applications:

• How will the material react when it is exposed to heat and elevated temperatures? Some materials will scorch, while others may crack if they are heated too rapidly or unevenly.
• Can the material withstand exposure to chemicals without a visual change or performance degradation? Exposure to acids or alkali, quite common in kitchens, can potentially cause etching.
• How stain-resistant is the material? It is important that the material does not stain easily and that stains can be removed easily. Materials with lower absorption rates tend to be more stain-resistant. Using a sealer on more porous materials can lower their absorption rate, helping them to perform better and be more stain-resistant.
• Is the material slip-resistant? Slip-resistance is the level of traction needed for a walking surface to remain safe for those using it. The texture, porosity, surface treatments, and elements including water, dust, and a build-up of cleaning product residue can alter the slip resistance of the material. Slip resistance is impacted by the material’s texture and porosity, as well as any surface treatments used and elements it is exposed to

Performance attributes should be addressed no matter what material you are considering or what type of project you are working on. These factors play an even more important role in sustainability and in the long-term performance of the material. Let’s look at how natural stone compares to some manmade materials from the perspective of these performance objectives.

Saratoga soapstone. Photo courtesy of Stone Store.

Interior Applications for Natural Stone versus Manmade Materials

Countertops

The possibilities for countertops using natural stone are quite extensive. Granite, quartzite, marble, and soapstone are among the many options to consider for these hardworking elements in a kitchen or bath. Every natural stone slab connects directly to nature as a piece of Earth’s history. These stones are versatile, meaning they can be cut and finished into different shapes and patterns, and with various finishes, creating a cohesive aesthetic. There are typically more options for edging and surface textures available for natural stone than manmade surfaces. Because most stone is formed in nature through extreme heat and pressure, natural stone is inherently heat resistant. Granite and quartzite are also very durable and scratch resistant. Marble is particularly prized for its beautiful veining, although it may be susceptible to etching. Although soapstone can scratch easily, it can also be easily repaired. Some stones are porous which may cause the material to stain over time. Sealers are recommended for most applications.

In some cases, natural stone may cost more than manmade materials initially—but often, natural stone proves to be the most cost-effective choice due to its durability.  This is especially true because natural stone can also be refinished, repaired, restored, or recycled.  Stone also has a low embodied energy, an important consideration for sustainability requirements. This is because natural stone was created by the Earth and very little energy or additional resources are needed to quarry, fabricate, and finish it.

Manmade materials including engineered quartz, porcelain, sintered surface, acrylic/solid surface and cultured marble are also options for interior countertops.  These materials typically do not need to be sealed. The patterns and colors of manmade materials are usually consistent within the same dye lot. Acrylic/solid surfaces are repairable, but it is often expensive to do so. Most of these materials do not last as long as stone. Porcelain can be more expensive to purchase and is more expensive to fabricate than natural stone. It is also difficult to repair since the patterns are not full bodied. These materials are typically made in thinner formats which also reduces the edge selection options. This results in the edges not having the same texture and pattern as the surface. The synthetic components of engineered quartz can break down when exposed to UV light. Acrylic, engineered quartz, and cultured marble are not heat or UV resistant. Variation between batches of manufactured materials may cause installation or customer service issues.  Most of these materials also have a high embodied energy because they require a wide range of other materials and energy intensive processes to be manufactured.

Photo courtesy of Coldspring.

Flooring

Natural stones including granite, travertine, limestone, marble, and slate are good options for flooring.  Again, one of the main performance aspects of using natural stone in this manner is its incredible durability. Design options abound with more edge treatments, surface texture, and color than most manmade materials. Most textures are slip-resistant and most natural stones are also UV resistant. Natural stone also comes in a variety of thicknesses and is easy to care for once properly protected.  They will also maintain their color and patina nicely over time if maintained properly.  In some cases, natural stone will cost less than porcelain or concrete imitations.

There are a few other issues to consider. Polished finishes may require a topical treatment to increase the slip resistance. Marble, limestone, and travertine can etch when exposed to acidic substances. Stones that have higher absorption rates will need to be sealed. Once again, the low embodied energy of natural stone makes it a great option for meeting sustainability goals. As previously mentioned, natural stone’s long life cycle makes it a very cost-effective choice.

Porcelain, ceramic, concrete, and terrazzo and plastic-based materials such as laminate and vinyl are among the most popular manmade flooring materials. A few of the pros of using these materials include the durability of ceramic and porcelain. Additionally, ceramic, porcelain, and vinyl are easy to care for. There are usage ratings available for specific applications, which helps make it easier to select the right option for the use you are considering. Terrazzo is a creative material because of the custom options available in production. Laminates are generally inexpensive while porcelain is more expensive than ceramic and other popular flooring materials.

There are a number of cons to using these materials. Porcelain is a heavy and unforgiving material, so specialized equipment is needed to cut it. The density of porcelain often requires that sub-floors and supports are strong enough to withstand the extra weight. These issues can also add to the cost of installation.  Most manmade materials are only manufactured in thinner formats and the edges do not have the same texture or pattern as the surface. Their patterns and sizing are limited and they typically cannot be refinished. Porcelain and ceramic are susceptible to chipping and laminates and vinyl can be easily scratched or indented. Laminate and vinyl also require adhesives that off gas, and laminates are not moisture resistant.  There will be variances across color between batches or lots of the materials. Many of the petroleum-based materials contain chemicals that are toxic, including those marked as Red List chemicals. These products also have high embodied energy, which creates a negative impact on the environment.

Galaxy Grey granite. Photo courtesy of Artistic Tile.

Natural Stone and Sustainability

Natural stone has demonstrated considerable durability in local and regional environments and applications. In many cases, these have lasted for hundreds of years, even millennia. Most of the manmade materials have not been around long enough to demonstrate that level of performance. The qualities of longevity and durability have proven that natural stone is a great choice for also meeting green building goals. The initial material and installation costs are off-set by the long life expectancy and the low maintenance requirements. Little to no cleaning or chemicals are required to maintain the material and as mentioned, natural stone can also be refinished, repaired, restored, or recycled. Natural stone is 100% natural stone. Engineered quartz, concrete, and porcelain require many different materials including chemicals to be manufactured and many of them are not recyclable.

So, as Oscar Wilde once said: “Imitation is the sincerest form of flattery that mediocrity can pay to greatness.” Choose natural stone and you will be contributing to the long legacy of great, high-performing, sustainable projects in the world.

Danby marble. Photo courtesy of Stoneshop.

SIMILAR ARTICLES:

Comparing the performance attributes of natural stone with manmade materials

Exterior Design

While design trends come and go, natural stone remains a timeless and flexible option for many exterior applications including cladding, paving, and hardscapes. This is due to the variety of natural stone types available that are beautiful, durable, and versatile. No wonder so many manmade materials try to emulate or replicate these characteristics and qualities.

It is important to know how the material chosen for an exterior project will perform under a variety of circumstances, conditions, and uses. It is also essential to understand how natural stone compares to manmade materials used for the same applications in order to select the material that will perform the best. At the same time, knowing how the material impacts the environment will ensure that you are choosing the most sustainable material for the application.

Photo courtesy of Connecticut Stone.

Performance Matters

A material’s optimal performance is based on a combination of the attributes of the material itself, how the material is installed, and the conditions to which it is exposed. There are many different issues to consider:

• How will the material hold up under freeze/thaw conditions? Continued cycles of freezing and thawing can cause some materials to degrade more quickly.
• Does it have good UV resistance and solar reflectance? Many materials will break down over time or change appearances when exposed to UV light.
• Does the material have a low absorption rate? Materials with lower absorption rates tend to be more stain resistant. Sealing a material can lower its absorption rate and improve its performance.
• Is the material slip-resistant? Slip resistance is the level of traction needed for a walking surface to remain safe for those using it. Slip resistance is impacted by the material’s texture and porosity, as well as any surface treatments used and elements it is exposed to, including water and dust.

Performance attributes should be addressed no matter what material you’re considering or what type of project you’re working on. These factors play an even more important role in sustainability and in the long-term performance of the material. Let’s examine how natural stone stacks up to manmade materials from the perspective of these performance objectives.

Photo courtesy of Gallegos.

Exterior Applications for Natural Stone versus Manmade Materials

Cladding and Veneer
The possibilities for natural stone cladding and veneer are endless. Limestone, granite, marble, travertine, basalt, slate, quartzite, and sandstone are among the many options. These stones are versatile, meaning they can be cut and finished into different shapes, textures, and patterns, creating a cohesive aesthetic. Many stones are durable in extreme weather conditions. When discussing vertical stone applications, the installation standards and techniques can make even vulnerable stones durable. Most stones have a long life span. Light colored stones have a high solar reflectance index, meaning they will reflect the sun well, reducing heat gain on the building or site. Stone also has a low embodied energy, an important consideration for sustainability requirements. This is because natural stone was created by the Earth and very little energy or additional resources are needed to quarry, fabricate, and finish it. Some stones are porous and may stain if subjected to contaminates over time. Depending on the stone’s mineral composition it may be susceptible to the effects of acid rain. Natural stone may sometimes have a higher initial cost than manmade materials, but it often proves to be the most cost-effective choice because it lasts so long. This is especially true because natural stone can also be refinished, repaired, restored, or recycled.

Manmade materials including porcelain, concrete, GFRC panels, metal, and terracotta are also options for exterior cladding.  Concrete and porcelain are durable and UV stable; good choices for the exterior and the exposure to sunlight and the elements they must withstand. However, these materials do not typically last as long as stone. Concrete, in the form of cast stone and cultured stone, is often tinted and can fade heavily from sunlight/UV exposure. Some manufacturers are improving the color fastness of cultured stone, but that adds chemicals and cost to the final product. Some manmade materials can also sometimes look flat since they are often made to be uniform in appearance and lack the natural veining and textures of natural stone. Most of them also have a high embodied energy because they require a wide range of other materials and energy intensive processes to be manufactured.

Paving
Natural stones including granite, sandstone, basalt, limestone, slate, travertine, marble, and porphyry are good options for paving. Again, one of the main performance aspects of using natural stone in this manner is its incredible durability. Surfaces can also be finished for slip-resistance and design options abound with more edging, pattern, surface texture, and color than manmade surfaces. Many natural stones perform very well in freeze/thaw environments and conditions. In some cases, natural stone will cost less than porcelain or concrete imitations. Once again, the low embodied energy of natural stone makes it a great option for meeting sustainability goals. The installation of natural stone may be more costly than some manmade materials and it may need to be sealed, depending on the type of stone, location, and environmental conditions. However, as mentioned previously, natural stone’s long life cycle makes it a very cost-effective choice.

Concrete, clay, and porcelain are among the manmade materials used for paving. A few of the pros of using these materials include the possibility of lower replacement and maintenance costs for concrete, and that porcelain is UV stable. There are limited texture options for concrete, which can contribute to a lower cost for the material. Quite often though, concrete fades and the material takes on a very aggregate-like appearance and will lose its color. New technologies are improving surface wear, color fastness, texture, and stain resistance. However, these add quite a bit to the cost and in some cases will make it more expensive than natural stone. For clay, the use of sand mold and wire cut production methods will yield a paver that has a high absorption rate and may have significant issues with salts and efflorescence. These pavers require a more costly installation process due to their larger sizing tolerance issues created by the moisture and firing process. There are pavers created with a compressed production process which perform better but still have performance issues and are not as durable as most natural stones.  High fired or vitrified clay pavers can be very hard and durable, but also very brittle. Porcelain behaves like glass and can also be very brittle, especially in a dry set application in a freeze thaw environment, so it will need to be set with spacers to protect itself in the winter. Porcelain has no absorption, so water will just sit on the surface. The water needs to evaporate or surface drain, so in the winter this can create a very slick surface and be dangerous. These products also have high embodied energy, which creates a negative impact on the environment.

Eden limestone dimensional flagstone. Photo courtesy of Lurvey Supply.

Natural Stone and Sustainability

Natural stone has demonstrated considerable durability in local and regional environments and applications. In many cases, these have lasted for hundreds of years, even millennia. Most manmade materials have not been around long enough to demonstrate that level of performance. The qualities of longevity and durability have proven that natural stone is also a great choice for meeting green building goals. The initial material and installation costs are offset by the long life expectancy and low maintenance requirements. Little to no cleaning or chemicals are required to maintain the material and as mentioned, natural stone can also be refinished, repaired, restored, or recycled. Natural stone is 100% natural. Engineered quartz, concrete, and porcelain require many different materials including chemicals to be manufactured and many of them are not recyclable.

So, as Oscar Wilde once said: “Imitation is the sincerest form of flattery that mediocrity can pay to greatness.” Choose natural stone and you will be contributing to the long legacy of great, high-performing, sustainable projects in the world.

SIMILAR ARTICLES:

​

Whether you’re using it for a countertop, fireplace surround, or flooring material, using natural stone is a way to let your personality shine. “Personality can be expressed in stone through color, design, shapes, and patterns,” says Susan Van Etten, owner of Murphy Marble Company and Galloy & Van Etten, Inc., a custom fabrication shop in Chicago. Each stone has its own personality. Some reflect stronger patterns while others feature more classic and subtle veining.

Trends in Natural Stone Flooring

Natural stone flooring is a favorite for many homeowners because of its aesthetic appeal, versatility, sustainability, and ease of care. It can be used both indoors and outdoors.

Photo courtesy of Carnevale & Lohr

“The beauty of natural stone is that it can last a very long time with just a little bit of maintenance,” says David Carnevale, president of Carnevale and Lohr. The company specializes in the supply, fabrication, and installation of natural dimension stone, including marble, granite, limestone, slate, onyx, and travertine. While most of the projects that his firm handles are commercial, he says it’s entirely possible to use any type of natural stone on any project since there are different categories that perform well. The goal is to find the right stone for the right application.

Photo by Carved Stone Creations, Inc. – More bathroom ideas

Carnevale’s clients often favor granite or quartzite for flooring because they tend to be impervious to acids and generally perform well in high traffic areas. That’s not to say that select types of marble or limestone won’t hold up to the wear from use. In fact, he notes that he’s installed floors with a white marble that’s denser than granite.

Van Etten’s firms also do a lot of restoration projects that often require stone be matched or replaced and they’ve worked on refinishing stone in historic or older buildings. Marbles are still popular among her clients, especially the classic white Statuary and Carraras, sometimes with an inlaid pattern as a checkerboard white with gray pattern. “Grays are very popular and considered a neutral,” she says. Black and white combos are also popular in both traditional and contemporary applications.

Another trend is toward larger sized pieces or pieces cut from slab. Van Etten is noticing more requests for mosaic inlays, with borders or patterns. Since her firm tends to do a fair amount of custom work, there are also more asks for bookmatching details, which must be done in full stone slabs.

One reason homeowners gravitate toward natural stone floor tiles is because they tend to be user-friendly and can be mixed and matched. They’re also easily replaced if one happens to crack due to an accident.

Photo courtesy of Galloy & Van Etten

Stone Floors That Stand the Test of Time

Photo by Felhandler/ Steeneken Architects – Browse entryway ideas

Since natural stone floors tend to be in high traffic areas, it’s important to educate yourself on which types of stones are best for these types of installations.

Natural stone can be textured, which is better for high traffic areas because it helps with slip resistance, according to Van Etten. She agrees with Carnevale that granite is a good choice for high traffic areas due to its durability, but also says that certain varieties of marble can be a viable option.

Carnevale notes that it ultimately comes down to understanding what the client wants and making sure they understand how to treat the stone. Finishes play an important role: honed, textured, and brushed finishes are popular, and matte finishes can help make the stone surface less slippery. If the client wants a high polish finish, this is possible, although Carnevale notes that this will require more regular maintenance.

“It just depends on the sensibilities of the clients,” Carnevale reiterates. In some cases, the client may want that Old World look and feel of natural stone, so they welcome the wear on it because it’s going to give the flooring a different kind of patina. In that case, he says, they won’t need to do a thing because the regular wear will accomplish that client’s goal.

Natural stone flooring isn’t restricted to high-traffic areas. For lower-traffic areas, softer stones like limestone or travertine can be a good choice.

Natural Stone Flooring As Part Of A Home’s Overall Design

“Refreshing or replacing flooring is often money well spent to improve your home’s appearance and functionality,” says Van Etten. Natural stone flooring complements other materials in the home, such as wood, as well as other natural stone applications.

Taking the time to consider the type of flooring you want will pay off in the long run. “You don’t want to invest in flooring that is outdated or doesn’t really express your personality,” says Van Etten. “This is your chance to think about what types of natural stone floors really catch your attention and make your house feel like your home.”

TAGS:  floor trends inspire megy karydes

SIMILAR ARTICLES:

How Do Your Countertop & Flooring Choices Impact the Environment?

The Life Cycle of Natural Stone vs. Engineered Quartz and Porcelain

By Stephanie Vierra, Assoc. AIA, LEED AP BD+C Vierra Design & Education Services, LLC | October 24, 2019 | Educate

Photo courtesy Rocky Mountain Stone.

So, you are ready to take on your next home design or remodeling project and have narrowed the material choices down to a few top contenders, such as natural stone, engineered quartz, and porcelain. The aesthetic qualities were probably among the first criteria you used to make your selections. But have you considered how each material impacts the environment? This information may not be provided by your material supplier. You have to dig a little deeper to find life cycle information, but it is important if you want your project to be eco-friendly and sustainable.

Left to right: Marble, quartz, and porcelain countertop options. They may look similar but each will have different impacts on the environment.

What Is Considered During a Life Cycle Assessment?

Different building materials have significantly different ingredients and processes that ultimately give them desirable performance characteristics like scratch, stain, and chemical resistance. Every option available for countertops and flooring has an associated life cycle as a result of the methods of production.  Understanding a material’s life cycle is an important component of the decision making process in today’s green building economy and should also be considered during your home design and renovation projects. Let’s take a look at some of the parameters used to determine a product’s life cycle before comparing the materials that made your short list.

A Life Cycle Assessment (LCA) systematically evaluates the environmental impacts associated with a building material or process. LCA-based information provides insights into the environmental impacts of raw material and product choices, as well as maintenance and end-of-product-life strategies. An LCA is very powerful and consumers can use it to more easily compare similar materials or alternatives. The life cycle may take into account some or all of the following stages of production:

• Raw Material Extraction
• Material Acquisition and Pre-processing
• Fabrication and Finishing
• Construction
• Installation
• Use and Maintenance
• End-of-Life

Why Does the Life Cycle Matter?

Life cycle information gives you the means to understand the impact of a product on the environment and your health. It addresses issues such as whether the material was created using fossil fuels or renewable resources; whether the material impacts global warming or ozone layer depletion; if there is waste generated during the process and how that is handled; whether there are toxic emissions from the material’s production, installation, use, and disposal; and whether the material’s creation, installation, and care requires chemicals and large amounts of resources. Choosing a material with a positive Life Cycle Assessment can ensure the health of your home and occupants.

Life Cycle Assessment in the Construction Sector. Graphic courtesy of UNESCO.

How Do You Select a Material Based on Life Cycle Information?

Today, many products have information about the life cycle or carbon footprint associated with them. This type of information is often available on the company’s website. The product may also have a sustainability label associated with it, such as GreenGuard or GreenSquared. Sustainably produced natural stone may be labeled with the ANSI/NSC373 Sustainability Standard. Access to products with these types of labels will help you more effectively compare the impacts across the same categories and criteria.

What are Some Differences between the Life Cycle of Natural Stone, Quartz, and Porcelain?

Let’s take a closer look at the life cycle of natural stone and compare it to engineered quartz and porcelain.

Connecticut Quarry Park

Natural Stone

The life cycle of natural stone begins when it is quarried out of the ground. The material is extracted from the quarry into blocks, and then transported to a nearby facility to be fabricated into slabs, tiles, or other custom sizes for use in a project. Energy and resources are used to quarry, transport, fabricate, and finish the stone. The energy consumed in this process is quite small compared to many other materials that are created from raw materials and may also include additives or chemicals. According to the Sustainability Study, Life Cycle Assessment of Floor Coverings, natural stone has a global warming potential that is 84% less than a large format ceramic/porcelain tile.

Photo courtesy of ILCO.

The natural stone industry has many measures in place to work toward constant improvements in all of the areas of environmental impact. Best practices include recycling water, reducing energy use for equipment and transportation, reducing packaging, and quarry reclamation plans that return the quarry to a natural or improved state. Natural stone typically is noted in most reports as having a 50-75 year lifespan. But it is possible for natural stone to last well over 100 years.

Natural stone can be recycled at the end of its first useful life, which is often well beyond 50-75 years, and is a consideration in the LCA in many cases. Recycling results in energy savings and less impact on the environment, because additional resources are not used to create new materials. (For more information on recycling natural stone see: Saving a Piece of History with Natural Stone.)

Engineered Quartz

Engineered quartz is manufactured through a factory-based process that uses quartz, resin, and other ingredients such as recycled glass, pigments, and additives. The life cycle of engineered quartz begins at the extraction of the materials used in its production, including the minerals and fossil fuels. Most of the materials used are considered to be virgin non-renewable resources and have negative impacts on health and the environment such as nitrates, dioxin, arsenic, lead, and mercury.  The typical lifespan of engineered quartz is between 25-30 years.

Energy and resources are considered in the LCA for engineered quartz. Unfortunately, that view does not consider recycling or reusing the material but instead assumes the material will end up in a landfill or get incinerated, which is an energy-intensive process, relies on high heat, and can also emit hazardous gases. When engineered quartz goes to the landfill, more glass, pigments, and additives must be mined, harvested, or refined to replace the discarded item.

Porcelain

The life cycle of porcelain includes seven stages: 1) mining the clay, 2) atomising the clay, 3) production of frits and glazes, 4) production of porcelain/ceramic tiles, 5) distribution, 6) installation and usage, and, on ending their useful life 7) treatment as construction and demolition waste. The main emissions into the atmosphere are related to the movement and grinding of raw materials which are dust and the burning of natural gas during the manufacturing process. The next largest environmental impact occurs when the clay is atomized and then during the distribution of the product. The lifespan of porcelain tile is typically 30-50 years.

Porcelain is also not recycled very often. Tiles are usually adhered with thin-set mortar or a type of epoxy. The mortar almost becomes part of the tile. Even if you can remove the tile, the bottom is very uneven and unsuitable for reuse. So again, resources are used to dispose of the material and to create new porcelain including the clays, stones, quartz, frits, and glazes.

Selecting a Material with a Good Life Cycle

There are many benefits of selecting and installing a material that has a favorable life cycle. First, you will know that your selection is not harmful to the environment or to the occupants of your home. Second, you will also be reducing material consumption by using a material that does not need replacing very often. You will also be choosing a material that can be recycled after its first useful life. This saves money and time over the life of your home and reduces further impacts on the environment.

Be vigilant in your research and ask a lot of questions. Request the data that you need to make an informed and educated decision. You will soon find that natural stone is the best choice for any project based on its low environmental impacts, longevity, and favorable life cycle.

Additional Resources

Declare Label

Green Guard

Green Squared

The International EPD System

International Organization for Standardization

Natural Stone Council

• ANSI/NSC 373 Sustainable Production of Natural Dimension Stone
• Granite Life Cycle Inventory
• Limestone Life Cycle Inventory
• Sandstone Life Cycle Inventory
• Slate Life Cycle Inventory

Stone Life Cycle

The EPD Study: Environmental Products Declaration

TAGS:  sustainability countertop educate floor stephanie vierra

SIMILAR ARTICLES:

Slate As A Building Material | History, Types & Common Uses

Many of us have had personal, perhaps desperate, moments with slate. The schoolroom blackboard was a proving ground of our formative years. When we were called to the front of the class to perform long division, diagram a sentence, or spell “Mississippi,” we gazed into the smooth, black void of the slate blackboard, hoping for a glimmer of divine intervention to illuminate a path to the solution.

The slate blackboard was introduced to American schools in 1801, at West Point Military Academy. In all their dusty glory, slate blackboards proved an enduring fixture in schools for nearly 150 years, until they were eventually replaced by ceramic-coated steel chalkboards.

In their heyday in the early 1900s, slate quarries in Pennsylvania, Vermont, Maine, and Virginia produced roofing tiles as their primary product. Slate shingles are naturally waterproof, fireproof, and durable. Depending on the grade of the stone, a slate roof can last from 70 years to over 200 years. But by the late 1940s, slate roofs gave way to modern asphalt shingles. Now, slate roofs are found mostly on historic buildings, museums, college campuses, or houses we can’t afford.

But slate is not out of our lives completely, thank goodness. It remains an especially useful stone due to its tendency to naturally cleave into thin layers. Slate has a consistent, dense grain that lends it durability. Slate is a beautiful choice for flooring, wall tile, or bathrooms. Slate can be warm and rustic, or sleek and modern. Either way, it’s trouble free and timeless.

Shale is made of flat-laying clay particles that settle out on the ocean floor.

What Gives Slate Its Traits?

Slate is a metamorphic rock. It starts out as marine mud, composed of clay, silt, and organic material that settles to the bottom of the cold, dark, ocean. There’s not a lot of action at the ocean floor. It takes millions of years just to accumulate a decent amount of mud. Over time, deposits of mud get blanketed by new layers, compressed, and hardened into shale. Shale is a sedimentary rock that forms in thin sheets and often contains fossils. Aside from the cool fossils, shale is not a particularly useful rock. Its layers are brittle, and the clay minerals are quick to weather apart.

But sometimes a layer of seafloor mud becomes trapped in a collision between continents. Then things get exciting, as the whole region crumples together, causing the rocks to become bent and folded, buried more deeply, and heated. This metamorphic process does not melt the shale.

Slate is made of compressed and recrystallized clay materials.

Instead, compression re-aligns the flat mineral grains, giving the rock a pronounced linear texture that often is at right angles to the original sedimentary layering. As the rock heats up, the minerals recrystallize and grow into each other, yielding a stone that is non-porous and sturdy.

Slate is fine-grained because the original mud particles were tiny. This makes slate smooth and sleek, as well as impermeable to water.

Camara Slate Quarry

Slate is commonly grey, but can also be black, green, purple, brown, or brick red. In most cases, the color of slate tells us something about the marine environment where the mud was deposited. Black and dark grey slates get their color from organic carbon. Red and purple slates are tinted by the oxidation of iron, which means that the ocean water was shallow or there were currents that mixed oxygen into the water. Green slate is the opposite of red – it’s caused by the reduction of iron in quiet or deep waters where little oxygen was available. A mottled, rusty tone on the surface of some slates is caused by groundwater that percolated along the natural seams in the rock, oxidizing the minerals along the way.

The Metamorphic Spectrum

The amount of heating and squeezing a given stone underwent depends on where it was in the tectonic collision zone. The more intense the heat and pressure, the more durable the stone became.

A slight amount of metamorphism creates a soft slate. Further compression produces a harder slate. As the rock is exposed to increasing heat and pressure, its minerals grow larger, and transform from dull clay to sparkly mica. At this point the stone is geologically termed phyllite. Phyllite shares its name with phyllo dough – as they are both made out of thin layers. (Phyllite is not nearly as tasty, and makes terrible baklava.) Phyllite looks similar to slate but has a sparkle or a metallic sheen to it. If the stone is heated even more, the mica minerals get bigger and the rock becomes glittery, creating a rock called schist.

There is no definitive point when shale becomes slate, slate becomes phyllite, and so on. It’s a continual gradation, depending on the conditions the stone endured. Thus you are likely to encounter stones from various points along the metamorphic spectrum. A general rule of thumb is that the degree of sparkle in a stone increases with more intense metamorphism. This “sparkle index” shows us that shale is dull grey, slate is slightly glossy, phyllite takes on a sparkly sheen, and schist is full-on glitter.

In the natural stone vernacular, the term “slate” refers to stones that willingly cleave along thin layers. At a stone yard or tile showroom, “slate” will include actual slate, along with phyllite and schist.

When selecting slate, you can use the idea of the metamorphic spectrum to help you identify stones that are suitable for your needs. Low-grade slate, which has not been subjected to much metamorphism, may split along its layers. Higher grades of slate are more robust; and phyllite is also strong and resistant to cracking. Schist can sometimes be troublesome because the larger mica grains are prone to peeling off. However, some schists have less mica and more quartz, and are exceptionally strong. Your best bet for evaluating a given stone is to examine the edges and look for signs of flaking. Also, note how much sparkle the stone has. A bit of shine is an indication of a stronger slate.

Uses of Slate Today

Today, slate is most commonly used as flooring. Most slate tile is split apart, rather than cut with a saw. This produces a slightly irregular “cleft” surface that adds texture and interest to a room. Slate’s colors are deepened with an enhancing sealer, which emphasizes the natural variations in the stone.

California Gold Slate. Photo Courtesy of M S International.

While slate was originally used mostly in the Northeastern US due to the proximity of slate quarries, slate’s popularity has spread nationwide. Quarries in the US still operate, and slate from India, Asia, and South America have also become commonplace.

At the Montana Tile and Stone showroom in Bozeman, Montana, owner Price Wills and fabricator Ari Kaufman praised slate flooring as carefree, durable, and in high demand. They reported that porcelain tile that is manufactured to look like slate has captured some of the market for flooring tile. But their allegiances clearly lie with natural slate. One advantage is that slate is easy to cut, shape, and trim, allowing for nearly limitless patterns. Unlike porcelain, the trimmed slate pieces are reusable elsewhere within a pattern, and “you can get creative with it,” explained Kaufman. Wills described “a push for authentic materials” among his clients. “You just can’t replace the real McCoy.” Since their showroom opened in 2001, natural slate flooring has remained a customer favorite, and is particularly popular for the rustic aesthetic of Montana homes.

Slate has evolved from a schoolroom staple, to fireproof roof, to a classic flooring material. In every application, slate is reliable and undemanding. After all, if it can last for millions of years on the ocean floor, it will serve you well on your own floor too.