Short answer: true quartzite scratches glass, isn't easily scratched by a knife, and doesn't etch from lemon juice or vinegar. Some stones sold as "quartzite" or "soft quartzite" are actually marble or dolomitic marble, and those fail at least one of the tests. Run both tests on a sample before the buyer commits, and you avoid the "my quartzite has etch marks" call after install.
All quotes below are from the Natural Stone Institute (NSI), checked 2026-10-29.
Why this matters
Buyers often pick quartzite because it looks like marble but is much harder. NSI says quartzite is "nearly twice as hard as glass and harder than the blade of a knife," and that it "will not etch when exposed to things like lemon juice or vinegar" (NSI).
The trouble is the label. NSI warns that "some stones are often mislabeled as quartzite or 'soft quartzite' when they are in fact marble or dolomitic marble," and quotes a fabricator: "We get calls from homeowners who think their countertops are quartzite, but they get etch marks" (NSI). As geologist Karin Kirk puts it in NSI's article, quartzite is "a stone with very consistent properties, but variable labeling."
The good news is that "this question is easily resolved with hardness and etching tests," and "anyone with a glass tile can easily tell the difference" (NSI).
Test 1: the glass scratch test
What you need: a glass tile and a piece of the stone with a sharp, broken edge.
Steps, from NSI's guide (NSI, "How to Be Your Own Stone Sleuth"):
- "Find a sharp, broken edge of the stone."
- "Place a glass tile on a tabletop, and then press the pointed edge of the stone into the glass."
- Drag the stone along the glass, pressing firmly but not so hard that the glass could break.
- "Then inspect the glass. Did the stone leave a true scratch that you can feel? Or did the stone simply 'draw' on the glass?"
How to read it: "Usually when the stone is scratching the glass you can feel and hear it biting in. When the stone is softer than the glass, it almost feels slippery as it glides across the surface."
On a full slab (with permission): NSI suggests the reverse. Use "the tip of a pocketknife, an awl, or similar pointy metal tool," press firmly and drag it across the stone, and "repeat the test multiple places, particularly if the slab is made up of different minerals or colors."
What the result means:
- Scratches glass, not scratched by metal: "Granite, gneiss, sandstone, and quartzite will all scratch glass and will not be significantly scratched by a metal tool."
- Doesn't scratch glass: "Marble, dolomitic marble, limestone, travertine, onyx, soapstone, and serpentine will not scratch glass."
So passing the glass test rules out marble, but it doesn't prove quartzite on its own. Sandstone passes too. On the Mohs hardness scale, quartz rates 7 and calcite rates 3 (USGS, checked 2026-10-29).
Test 2: the acid (etch) test
What you need: lemon juice or white vinegar, a cloth, and a light you can look across at a low angle.
Steps, from NSI's guide (NSI):
- "Put several large drops of lemon juice or vinegar on the surface of the stone and leave it there for 5-10 minutes."
- Watch it. "Sometimes you can see a faint bubbling or fizzing on an acid-sensitive stone, but you might have to use a magnifier to see this."
- "Wipe down the stone and look at it at a low angle."
How to read it: "Etched areas can look dulled, as if the stone's polish has been removed." NSI notes that "polished slabs typically will have more noticeable etching than honed or leathered slabs," so look carefully on a honed sample.
What the result means: "Stones that contain calcite or dolomite will etch from contact with common acids." True quartzite won't.
Sealer doesn't change either test
A common question from buyers: "Maybe it's just not sealed?" NSI is clear on both tests: "Sealer will have no effect on hardness tests," and "sealers do not protect a stone from interactions with acid, so this test can be done on a sealed or unsealed sample" (NSI).
Running the tests in a showroom or yard
- Get permission if you don't own the slab. NSI: "If you don't already own the slab, make sure you get permission before doing this!"
- Use a sample or offcut where you can. The etch test is meant to leave a mark on a stone that etches, so don't do it on a finished face the buyer will see.
- Test several spots, especially on slabs with mixed colors or minerals.
- Write down the result with the slab or lot, so the buyer and the shop are working from the same facts.
How to explain it to a buyer
Keep it simple and show them:
"Quartzite is hard enough to scratch glass, and lemon juice won't dull it. Marble is softer, and acid etches it. Some stones get sold as 'quartzite' or 'soft quartzite' when they're really marble. Let's test this one, so you know exactly what you're getting."
If the stone etches, that doesn't make it a bad choice. It makes it a marble, with marble care. Some buyers want that look and accept the etching. They should choose it knowing that.
For the wider comparison (sealing, heat and silica safety in the shop), see quartz vs granite vs quartzite.
Where a visualizer fits, and where it doesn't
A countertop visualizer, ours included, shows what a color looks like in the buyer's kitchen. It can't tell you what a stone is. A quartzite and a marble with the same look will appear the same in a visualization, so the label in your catalog is only as accurate as your supplier's, and these tests are how you check it.
In Countertop Visualizer, the salesperson photographs the buyer's kitchen, taps a slab from your catalog and sees it on their counters in about 20 seconds. The tests above then confirm what the buyer is choosing. 7 days free, then $250/month.