Are you ready for a lesson in Geology that really rocks? We hope to give you just that by teaching you about the rocks in your countertops. We will be discussing three main types of natural rock formations involved in common countertop materials: igneous, metamorphic, and sedimentary.


Igneous Rock


If you have granite countertops, you are familiar with igneous rock whether you knew it or not! Today, granite is the most commonly utilized igneous rock. Granite is an extremely dense and durable material. Furthermore, each slab is striking in its own unique way. Combine its beauty with its durability and there is no wonder why it is such a popular choice for countertops. However, you may be wondering how granite went from a rock to the countertops that you love.

Granite was formed hundreds of millions of years ago from scorching magma buried deep within the earth’s crust. Geologists have several theories of how the granite travels towards the surface from miles within the earth. To summarize them, forces within the earth thrust the granite up towards the surface.

The color of granite is determined by the element and mineral composition. The more slowly the granite cooled during formation, typically the lighter it will be. If it cooled very quickly, it will be a darker color. Feldspar makes up at least half of the rock and gives granite its background color. Quartz is responsible for between 25-40% of granite’s composition. Quartz is often medium darkness and has a foggy transparency to it. Mica only accounts for about 10% of granite’s makeup and is typically opaque and dark.

Metamorphic Rock

Do you have marble, quartzite, or soapstone countertops? If so, you are familiar with this rock! Metamorphic rock results from a transformation of existing rock types through heat and pressure. This heat and pressure triggers a metamorphism (hence the name metamorphic) that alters the composition of the existing rock.

Marble has an interesting journey. It is formed when either limestone or dolomite, both sedimentary rocks, go through the metamorphism. Although marble has similar characteristics to granite, its origin causes it to have a different density, making it less durable than granite. As you probably have noticed, it has a different appearance than granite as well. The coloring of marble varies depending on minerals present during metamorphosis. If only calcite (essentially pure limestone) is present, the marble will be predominately white. If other minerals are present, the marble can pick up shading or coloring. Now, lets learn about the minerals that give certain slabs their vibrance! The presence of hematite gives marble a red hue, whereas limonite causes a yellow coloring. Graphite will add a black tint and serpentine provides a green tone. If you prefer bold colored granite, you have these minerals to thank. Furthermore, the characteristic swirls and veining found in marble originate from other substances that are present during the formation, including: sand, clay, silt, and iron oxides.

Soapstone, another type of metamorphic rock, is a talc-schist. It is produced by dynamothermal metamorphism and metasomatism. Now that sounds pretty complex, so what does it mean? Soapstone is created by heat and pressure, with influx of fluids, but without melting. Otherwise, your hopes and dreams of a soapstone countertop may be crushed, as it would basically be lava. Because it is primarily composed of magnesium and talc, it has limited color variation. Its typical color options are black, gray, and bluish/green gray. The composition of soapstone also causes it to be a softer material.

Quartzite is a metamorphic rock formed when quartz-rich sandstone or chert has been exposed to heat and pressure that fuses the quartz grains together. As the name suggests, this rock is made up of between 90%-99% quartz. Quartzite is a dense, hard, equigranular (meaning it is composed mainly of crystals) material that offers great durability. It typically is white to gray in color. However, some rock units are stained by iron. This can make the stone appear pink, red, or purple. Other impurities can cause quartzite to be yellow, orange, brown, green, or blue. depending on the impurities present, quartzite has a wide range of color options.


Sedimentary Rock

The most commonly known example of this rock is limestone! Unlike the igneous rock, sedimentary rock forms both underwater and on the earth’s surface. Sedimentary rock is ultimately composed of multiple layers of sediment built up over time. Because it lacks the heat and pressure that creates igneous rock, it does not have the same density and is therefore less durable.

You have probably seen limestone countertops at some point. However, unlike granite, limestone is not a very popular countertop material choice. Although limestone is well resistant to  crumbling or deteriorating, it is not as dense as granite. For this reason, it will absorb liquids, stain easier, and scratch easily in a kitchen application. Cladding the interior and exterior of buildings, paving walkways, and landscaping are all popular uses of limestone.


Congratulations! You have made it through your first Cameo Course. Stay tuned, we have more lessons planned for you!