Use Natural Stone is a great online resource for learning everything natural stone. Here’s a great article we found on a Guide to Quartzites. It answers a lot of commonly asked questions about one of our favourite Brazilian stones.
Read the full article, The Definitive Guide to Quartzites by Karin Kirk here.
The Definitive Guide to Quartzite
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 favourite tool – geology – to suss out the answer to this mystery.
I’ll get right to the point: quartzite is commonly mislabeled. Some quartzite is the real deal, but sometimes marble or dolomitic marble are labelled as quartzite. Because each of these stones behaves differently, people might understandably conclude that quartzite is variable. But it isn’t; quartzite has very consistent properties. Unfortunately, it has variable labelling.
Emerald Green Quartzite at VSG.
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 neighbours, forming a dense, durable rock. The process is similar to individual snowflakes merging into solid, glacial ice.
Quartzite is usually white or light-coloured because quartz sand is light coloured. Additional minerals carried by groundwater can impart hues of green, blue, or ion-red. Van Gogh and Azul Macaubas quartzites are examples of vivid colouring.
Regardless of colour, quartzite is made of one thing: quartz. That’s helpful because quartz has distinct properties that make it easy to tell apart from other minerals. (Note I’m talking about the mineral quartz, not the composite countertop material that is also named quartz.)
Properties of quartzite
You needn’t be a geologist to appreciate the hardness and durability of quartzite. Not only does this make for a tough stone, but it also makes it easy to tell quartzite from the imposters. Quartz is 7 on the 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 labelled as quartzite is soft, then it was mislabeled. The unfortunate term “soft quartzite” has emerged to try to explain why a rock that is labelled 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 labelled as soft quartzite is most likely marble.
Resistance to acids
Quartzite will not etch from acids like lemon juice or vinegar. If a rock labelled 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!
Quartzite has a range of porosities. Some, like Taj Mahal or Sea Pearl, have been highly metamorphosed, and the minerals 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.
Things that do not help distinguish quartzite from non-quartzite
The country of origin, price, or the name of a stone are not reliable indicators of what type of rock you’ve got. In many cases, it’s not even possible to tell marble and quartzite apart visually.
What’s the difference between quartzite and granite?
Granite is a whole separate category of rocks that form from liquid magma. Visually, granite has distinct flecks of darker colours in it, while quartzite has either no dark colours at all or has subtle, flowing areas of different colours.
Sometimes quartzite is mislabeled as granite, which is not the worst mistake because they have similar properties. Granite and quartzite are both harder than glass, and neither will be etched by acids. Geologically, they are different classes of rocks, but that is less important than how they will behave on a countertop or as floor tile.
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