Clear and white stones cause more identification headaches than almost anything else. Quartz, calcite and glass can look nearly identical at first glance, especially as broken pieces or tumbled pebbles, and all three turn up on beaches, in gravel and in rock shops. Fortunately each has a few properties the others don't, and you can test most of them at home in a few minutes.
Quick comparison
| Property | Quartz | Calcite | Glass |
|---|---|---|---|
| What it is | Mineral (silicon dioxide) | Mineral (calcium carbonate) | Man-made or volcanic, not crystalline |
| Mohs hardness | 7 | 3 | About 5.5 |
| Scratches glass? | Yes | No | Only harder glass, weakly |
| Scratched by a knife? | No | Yes, easily | Yes, with effort |
| How it breaks | Conchoidal fracture, no cleavage | Perfect cleavage into rhombs | Conchoidal fracture |
| Crystal shape | Six-sided prisms with pointed tips | Rhombs, "dogtooth" points, many forms | None, unless shaped by moulding |
| Double image through it | Not visible | Strong in clear pieces | No |
| Fizzes in acid | No | Yes | No |
| Bubbles inside | Rarely round | Rarely round | Common, perfectly round |
| Luster | Glassy (vitreous) | Glassy to pearly on cleavage faces | Glassy |
The sections below explain each test and how to read it.
1. Hardness: the fastest test
This is the single most useful test for these three.
- Quartz (7) scratches glass easily and can't be scratched by a steel knife.
- Calcite (3) can't scratch glass and is easily scratched by a knife or even an old copper coin.
- Glass (about 5.5) is scratched by quartz and, with effort, by a hard steel blade.
Try scratching a piece of window or bottle glass with your stone, pressing firmly. Then wipe the spot and check with a fingernail for a real groove. A powdery line that wipes away is your stone leaving its own powder behind, which means it's softer than the glass. Our guide to the Mohs hardness scale at home explains how to avoid that and other common mistakes.
2. Cleavage vs conchoidal fracture
How a material breaks reflects its internal structure.
- Calcite has perfect cleavage in three directions. It breaks along flat, shiny planes into slanted blocks called rhombs, a bit like a squashed cube. Even a broken calcite pebble often shows stepped, flat, mirror-like faces.
- Quartz has no cleavage. It breaks with conchoidal fracture: smooth, curved, shell-like surfaces, often with fine ripples.
- Glass also breaks conchoidally, because it has no crystal structure at all.
So if you see neat flat planes meeting at angles that aren't 90 degrees, think calcite. If you see curved shell-like breaks, it's quartz or glass, and hardness or bubbles will decide which.
3. Crystal shape
If your specimen still has natural crystal faces, the shape alone is often enough.
- Quartz typically grows as six-sided prisms topped with a six-sided point. The long prism faces often have fine horizontal lines (striations) running across them.
- Calcite forms in many shapes, more than almost any other mineral. Common ones include rhombs, sharp "dogtooth" points (scalenohedra), and flat or nail-head shapes. Its crystals are never the neat six-sided prism and point of quartz.
- Glass has no crystal form. "Crystal points" made of glass are cut or moulded, and often look too perfect, with identical faces and very sharp, even edges. Look for mould seams.
4. The double refraction test
Clear calcite has one of the strongest double refraction effects of any common mineral. Light entering it splits into two rays, so anything you look at through it appears doubled.
Place a clear piece of calcite on a printed page or a single line drawn on paper. If the text appears doubled, you're almost certainly looking at calcite. Turn the stone and one of the two images will rotate around the other. Very clear calcite with this effect is called Iceland spar.
Quartz also splits light, but so weakly that you can't see a doubled image with the naked eye. Glass doesn't do it at all. This test only works on fairly transparent pieces; milky or white calcite won't show it.
5. The vinegar fizz test
Calcite reacts with acid and gives off carbon dioxide. Geologists use a few drops of dilute hydrochloric acid in the field, but household white vinegar works as a gentler substitute.
- Choose a small, inconspicuous spot, or scrape off a little powder onto a plate.
- Add a few drops of vinegar.
- Watch closely, with a magnifier if possible, for tiny bubbles forming on the surface.
Calcite will fizz. With vinegar the reaction is slower and gentler than with stronger acid, so be patient and look carefully; powder reacts more visibly than a solid surface. Quartz and glass won't react at all.
A couple of cautions: the acid will etch calcite, leaving a dull spot, so don't do this on a specimen you want to keep pristine. Dolomite, a similar mineral, fizzes only weakly in cold acid, and mostly when powdered. Rocks made of calcite, such as limestone and marble, will also fizz.
6. Look for bubbles
Round bubbles are the classic sign of glass. Molten glass traps air, and the bubbles stay perfectly spherical when it cools. Glass may also show swirls and flow lines.
Natural quartz and calcite form slowly from solutions or melts and have very different inclusions: irregular or angular cavities, feathery cracks, cloudy zones, and tiny crystals of other minerals. Some quartz contains small fluid inclusions with a moving bubble inside, but these sit in irregular cavities rather than floating as neat round spheres.
Use a 10x loupe and bright light. A few perfect spheres are strong evidence for glass.
7. Two extra clues
- Temperature. Quartz conducts heat better than glass, so a quartz piece often feels slightly colder to the touch at first. It's a subtle difference and only a supporting clue.
- Where you found it. Beach glass is common near towns, old dumps and shipping routes, and has a frosted, pitted surface. Calcite is common in limestone areas, in cave deposits and as white veins in many rocks. Quartz is everywhere, from veins in granite to sand grains on the beach.
Putting it together
Most of the time, two tests settle it:
- Does it scratch glass? If yes, it's almost certainly quartz.
- If not, does it fizz in vinegar or show flat rhomb-shaped cleavage? If yes, it's calcite. If neither, and especially if it has round bubbles, it's glass.
Tip: if you're testing several clear pieces at once, label them first. Small white and clear fragments are very easy to mix up halfway through.
AI Stone can suggest from a photo which of the three you're most likely holding and lists each one's expected hardness, cleavage, fracture and refractive index, which are the properties these tests check. A photo can't measure hardness or reactivity, so use the scratch and fizz tests to confirm it. For a wider set of tests on other stones, see how to identify a rock, and for crystals that may be imitations, real vs fake crystals.