Telling White Stones Apart
How to Tell If Your White Stone is a Granite, Quartzite, or Marble
One of the favorite parts of my job as a roving geologist is to share not just an appreciation for natural stone, but a deeper understanding of it. With a little guidance and practice, anyone can learn to recognize the properties and aesthetics of various types of stone. No matter a slab’s label, country of origin, or price tag, some basic geologic savviness can help us understand it better.
The process of identifying stones gets most confusing when they look alike, and nowhere is this more vexing than with white stones, which are popular, frequently mislabeled, and often misunderstood. But fear not. Even similar-looking stones can be sorted out by using a few basic guidelines.
Read on, as we step through the process of differentiating light-colored granites, quartzites, and marbles.
Granite has a distinct look, compared to quartzite and marble.
Granite is the most common type of natural stone in the trade and comes in a satisfyingly large range of colors and patterns. Despite their diverse aesthetics, granites have common elements that make them recognizable. Below are some rules of thumb. Please note that I’m referring to the general industry classification of granite, rather than the narrower geologists’ definition.
- Light colored granites have flecks or blocks of varying colors. White granites are rarely a homogeneous, even-toned color. Most light colored granites have more than one color in them, and it’s rare to have a white granite without any darker minerals at all.
- Individual minerals are visible. Look for blocky crystals of feldspar, glassy areas of quartz, and a smattering of darker colored minerals. Learn more about what different minerals look like.
- Sometimes granite has giant minerals! If the slab has minerals larger than couple of inches, then you’ve got a pegmatite. That’s special type of granite with super-sized crystals. Patagonia is one of the more dramatic examples of a granite pegmatite, and Tourmaline, Alpine, Alaska White, and Delicatus are other examples.
- If it contains garnet, it’s granulite or gneiss. Garnets are small, round minerals that are dark pink, burgundy, or reddish brown in color. Their presence is an instant giveaway that you’re looking at metamorphic variations of granite. Gneiss has stripes or bands of lighter and darker minerals, while granulite tends to have few or no stripes and is generally light colored overall. There are many white granulites on the market, including Bianco Romano, Colonial White, or Giallo Cream. Viscount White is an example of gneiss.
- Mica is common in granite. Mica is present in small amounts in granite, and it makes an appearance as glittery minerals that can be silver, gold, bronze, or metallic black. If a stone is mostly made of mica, then it’s schist.
- Granite can have quiet patterns or vivid movement. Leave it to Mother Nature to create waves of color, veins of contrasting minerals, and all sorts of other interesting effects. That’s a big part of the appeal of a natural stone.
- Read more about the variations, colors, and origins of granite.
Marble and quartzite look different from granite, but similar to each other.
Marble and quartzite look alike in several ways.
- They tend to be mostly light colored: white, light grey, cream. Darker colors are also possible.
- Marble and quartzite are usually fine grained overall; you can’t put your finger on an individual mineral grain.
- They often have layers or bands of contrasting colors. The layering can be straight, wavy, or chaotic.
- Slabs can have quiet patterns or vivid movement.
- If a slab has fossils, then it’s limestone, not marble. Limestone has a chalky or matte finish compared to either marble or quartzite.
- Read more about marble and quartzite.
To tell marble from quartzite, check their properties.
The fact that these two stones look alike is why they are so frequently mislabeled and misunderstood. Your best bet is to rely on the way the stones behave rather than how they look. Thankfully, marble and quartzite have consistent traits that you can use to tell them apart.
- Quartzite is much harder than marble and slightly harder than granite.
You can use a glass tile or a knife blade to gauge the hardness of a stone. Quartzite will leave a definite scratch on glass, and a knife blade will not easily scratch the stone (though it may leave a silvery mark on the stone, which is the metal rubbing off on the stone). Learn more about identifying quartzite in the Definitive Guide to Quartzite.
- Marble is relatively soft.
A knife blade will make obvious scratch marks on marble; and marble will not scratch glass. Some marbles, like Super White or Fantasy Brown, may have isolated pockets of quartz amid an overall composition of marble. When in doubt, check a few different areas of the stone.
- Quartzite is not affected by acids.
Vinegar, lemon, wine, citrus, or any other common acid won’t affect quartzite. Strong chemicals like oven cleaner or rust remover can damage almost any stone, though, so be careful with those.
- Marble is acid-sensitive.
The mineral calcite is dissolved by acids, and many types of stone contain calcite, such as limestone, onyx, travertine, and marble. (Science trivia: this is why acid rain caused damage to buildings and monuments before we got a handle on the pollution that causes acid rain.) Dolomite is a mineral that is chemically similar to calcite, and it’s also affected by acids but not as quickly. In either case, acids leave an etch mark or a dull spot on the surface of the stone. Etches do not affect the structural integrity of a stone, and they can be polished out if need be. For people who love marble, etches are often considered a part of the living surface of the stone, which develops depth and character over time. If that idea makes you cringe, then you know marble is not for you. Choose quartzite or granite instead!
Granite, marble and quartzite can be porous…or not.
Some types of stone have small pore spaces within the stone, and a porous stone can absorb liquids and become stained. There are several geologic processes that affect a stone’s porosity. For example, if a stone does not get buried too deeply when it forms, tiny spaces can remain between mineral grains. Another possible cause of porosity is underground fracturing along fault lines where rocks grind up against each other. Sometimes groundwater dissolves away pieces of the stone. On the other hand, groundwater can also do the opposite – it can add bits of minerals that will fill in pore spaces.
So we just have to live with the fact that sometimes granite, marble, and quartzite are slightly porous, and sometimes they are not. That also means that porosity cannot be a guide to helping us identify a stone. We can’t say that a stone is porous therefore it’s marble, for example. Because plenty of marbles are dense and impermeable. The same is true for granite and quartzite, too.
The porosity of quartzite can be especially confusing, and you can learn more in the Deep Dive Into the Properties of Quartzite.
So while rules of thumb fail us, what we can do is test slabs to learn about their porosity – which is actually ideal, because you’ll get specific details on the slabs of interest.
Refer to How to Be Your Own Stone Sleuth for instructions and photos showing how to test your slab’s hardness, acid sensitivity, and porosity. Nothing can beat the hands-on information you’ll get from doing a little “sciencing” on various slabs, and it’s pretty fun, too.
So don’t let the array of white stones get the better of you. By learning a bit more about how different stones get their particular aesthetics and properties, you’ll be able to enjoy them all the more.