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14|April 2020
The Definitive Guide to Quartzite
©2017 by Karin Kirk for MIA+BSI
Slippery rock GAzette
Quartzite may be the most confusing natural stone out there. A quick internet
search will reveal an unsettling disparity of information. Some people say it etches. Some say it doesn’t. Sometimes you hear it’s a hybrid between marble and granite. Yet, others report that it’s harder than granite. Which is it? Why are there such conflicting reports about quartzite?
Let’s use my favorite tool – ge- ology – to suss out the answer to this mystery.
I’ll get right to the point: quartz- ite is commonly mislabeled. Some quartzite is the real deal, but sometimes marble or dolomitic marble are labeled as quartzite. Because each of these stones be- haves differently, people might understandably conclude that quartzite is variable. But it isn’t; quartzite has very consistent properties. Unfortunately it has variable labeling.
What is Quartzite?
Quartzite is a metamorphic rock made almost entirely of the mineral quartz. Quartzite begins its geologic life as sand grains, perhaps on a beach, desert dune, or riverbed. Over time, the sand grains become compressed and stuck together to form sandstone. If the sandstone gets buried ever more deeply underneath layers of rocks, it gets hotter and more compressed. With enough heat and pressure, the sand grains lose their original shape and fuse to their neighbors, forming a dense, durable rock. The process is simi- lar to individual snowflakes merg- ing into solid, glacial ice.
Karin Kirk
usenaturalstone.com
Diagrams and photos by Karin Kirk and courtesy Natural Stone Institute
Quartzite is usually white or light-colored because quartz sand is light in color. Additional minerals carried by groundwater can impart hues of green, blue, or iron-red. Van Gogh and Azul Macaubas quartzites are examples of vivid coloring.
Regardless of color, quartzite is made of one thing: quartz. That’s helpful because quartz has dis- tinct properties that make it easy to tell apart from other minerals. (Note I’m talking about the min- eral quartz, not the composite countertop material that is also named quartz.)
Properties of Quartzite
Hardness
You needn’t be a geologist to appreciate the hardness and dura- bility of quartzite. Not only does this make for a tough stone, but it also makes it easy to tell quartzite from the impostors. Quartz is 7 on Mohs hardness scale. That means it’s harder than glass and harder than a knife blade. These things are easy to test with a sample of stone.
If a rock that is labeled as quartzite is soft, then it was misla- beled. The unfortunate term “soft quartzite” has emerged to try to explain why a rock that is labeled quartzite is actually not hard and durable like real quartzite. There is no such thing as soft quartzite though. There is only one kind of quartzite and it’s hard. A rock la- beled as “soft quartzite” is most likely marble.
Resistance to Acids
Quartzite will not etch from acids like lemon juice or vine- gar. If a rock labeled as quartzite becomes etched from acid, then it’s been mislabeled. Marble and dolomitic marble, on the other hand, will etch from these acids. Dolomitic marble etches slightly more slowly than regular marble. But quartzite will not etch at all from normal kitchen acids. Not one little bit!
Porosity
Quartzite has a range of po- rosities. Some, like Taj Mahal or Sea Pearl, have been highly metamorphosed, and the miner- als are bonded together tightly. White Macaubas and Calacatta Macaubas have been exposed to less intense pressure, so they are more porous and will benefit from sealing.
How do you know if it’s real quartzite?
I know what you’re thinking. If the rock is sometimes mislabeled, then how do you know what it ac- tually is? Thankfully, there is an easy test that anyone can do. It’s called the glass test.
Who should do the glass test? Everyone. People who sell quartzite should do it because they can easily find out if the rock is correctly labeled, and they can change the labeling if needed. Consumers should also do the glass test because not every showroom has read this
article. It’s worthwhile to learn as much as you can about any stone you are buying (or selling).
• Use a glass tile.
• Find a rough section of the
stone, preferably a pointy edge. • Put the glass tile on a table,
then try to scratch the tile with
the stone. Press hard.
• Inspect the scratch. Is it really a
scratch? Or is it a powdered trail of crumbled rock?
If the stone is variable and has more than one color or type of mineral in it, repeat the test in different places.
What the results mean:
Real quartzite will scratch glass easily. You’ll hear it grind and you’ll feel it bite into the glass. The resulting scratch will be obvi- ous. Yeah! You’ve got quartzite!
Non-quartzite will either leave no scratch or a very faint scratch. Often the rock feels slippery against the glass. It doesn’t make a noise. It leaves a powdery trail that rubs right off. Darn. Not quartzite. But aren’t you glad you know that now?
What if there is no broken edge to use, such as a full slab with ep- oxied edges?
You can do a similar test with a knife blade. Try to scratch the rock with the tip of the blade. Genuine quartzite will be scratched lightly or not at all. Marble or dolomitic
marble will be easily scratched.
You can also try the etching test.
Put lemon juice or vinegar on the surface of the slab, and let it sit for 15 minutes or so, and then wipe it off and see if the stone is affected. Etching can look like a lighter area, a darker area, or a place where the shiny finish of the stone has become dulled. When exposed to acids like vin- egar or lemon juice, marble will show obvious etching. Dolomitic marble will show more sub- tle etching. Quartzite will have zero etching. Note that strong acids like in rust stain remov- ers can etch even granite and quartzite. For our purposes, we are talking about everyday acids you’d cook with: tomato sauces, balsamic vinegar, etcetera.
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The knife blade test is a simple and easy way to determine if that slab is marble of quartzite.
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— P. J. O’Rourke