How to Identify a Stone Yourself
Two simple tools tell you exactly what stone you have. The whole protocol takes less time than reading this article.
Why diagnostic tests exist
Stone is widely mislabeled in the trade. Country of origin gets passed along by word of mouth. Stone names are inherited from importers who didn't verify them. Visual inspection isn't reliable — quartzite and marble can look almost identical to the eye.
The good news is that the underlying minerals behave dramatically differently when challenged with the right stimulus — and those stimuli (a piece of glass, a drop of acid) are cheap and quick.
The Mohs hardness scale
Mohs is a scale of mineral hardness from 1 (talc) to 10 (diamond). The numbers that matter for stone identification:
- 3 — Calcite. Limestone, marble. Soft enough to be scratched by a copper coin.
- 3.5 — Dolomite. Dolomitic marble.
- 5.5 — Glass and most knife blades. The reference threshold.
- 6 — Feldspar. Granite (mostly feldspar with some quartz).
- 7 — Quartz. Quartzite. Harder than glass.
The four-point gap between marble (3) and quartzite (7) is what the glass scratch test exploits.
Test 1 — Glass scratch
Find a rough, un-epoxied edge on the slab — usually the back, a saw cut, or a corner. Press it firmly against a glass tile and try to scratch the glass.
- Quartzite or granite: obvious scratch. The stone bites into the glass and grinds audibly.
- Marble, limestone, travertine, onyx: no scratch, or a powdery trail that wipes off. The stone feels slippery against the glass.
- Inconsistent results: some areas scratch, others don't. This is usually marble with minor quartz inclusions — still treat it as marble.
Knife alternative: if the slab has no exposed rough edge, drag a knife tip across the surface in an inconspicuous spot. Real quartzite resists. Marble gouges easily.
Test 2 — Acid drop
Place one drop of 5–10% hydrochloric acid on an unsealed surface. Watch for bubbles for 10–30 seconds. If you don't see anything, look inside the drop with a magnifier — sometimes the bubbles are subtle.
- Bubbles, fizzing: the stone contains calcite. It is marble, limestone, or travertine.
- No reaction: the stone may be quartzite or granite. Confirm with the powdered rock test below.
On polished surfaces, the acid drop also leaves an etch mark — a small, dulled spot — that confirms calcite content definitively. Rinse and dry, then inspect for the etch.
Test 3 — Powdered rock (catches dolomitic marble)
Dolomitic marble (the "Super White" problem) reacts very slowly with acid on a solid polished surface. To force a reaction, increase the surface area:
- Use a knife to scratch a small powder pile on the slab.
- Leave the powder in place.
- Place a drop of acid directly on the powder.
- Watch closely.
- Bubbles in the powder: the stone is dolomitic marble. It looks like marble, behaves like marble, and should be sold as marble.
- No bubbles, even in powder: congratulations, you have real quartzite (or granite — the glass test sorts those two apart).
Decision tree
| Surface acid drop | Powder acid drop | Identity |
|---|---|---|
| Bubbles vigorously | — | Limestone or marble (calcite) |
| Bubbles weakly | — | Dolomitic marble |
| No reaction | Bubbles weakly | Dolomitic marble |
| No reaction | No reaction | Quartzite (or granite — confirm with glass test) |
Don't have a kit handy?
Try the AI photo identifier instead
Upload a photo of the slab and our AI returns the three most likely matches from our catalog of 786 stones with confidence scores in 10–20 seconds. Less authoritative than a glass + acid test on an unsealed edge, but a useful first pass when you can't do the manual diagnostic.
Open the identifier →The Stone ID Kit
Stone Intelligence ships a Stone ID Kit with the diluted HCl dropper bottle, glass tile, and other implements you need to run these tests reliably. The acid is pre-diluted to the correct strength — strong acids will etch even granite, so getting the dilution right matters.
Bring it to slab yard appointments. Ask the salesperson to pick a hidden corner. Run the tests. Take photos of the results. You'll spend less than $5,000 on a slab and you'll know exactly what you're getting.
Shop the Stone ID KitA note on safety
Even diluted hydrochloric acid is corrosive. Wear gloves and eye protection. Work in a ventilated area. Strong acids — like rust-stain removers from the hardware store — will etch even granite and quartzite, so don't substitute. The 5–10% dilution is the standard geological-test concentration and is what you should use.
Next step
See how each stone in our library is classified and what tests apply to it. Browse the stone library or read Quartzite vs Marble for the most common identification problem in the trade.