How can you tell if something is quartz or glass?

The easiest way is the scratch test: quartz (hardness 7) scratches glass (hardness 5.5), but glass cannot scratch quartz. You can also check for bubbles inside (glass has them, quartz doesn't), feel the temperature (quartz stays cool longer), and compare weight (quartz is denser).

Does real quartz scratch glass?

Yes. Quartz has a hardness of 7 on the Mohs scale while glass is about 5.5. Quartz will easily scratch glass. If your specimen cannot scratch glass, it's likely not quartz.

Can quartz have bubbles inside it?

Natural quartz very rarely has round air bubbles. If you see perfectly round bubbles inside a clear stone, it is almost certainly glass. Quartz may have inclusions (other minerals, cloudiness, fractures) but not smooth round bubbles.

Quartz vs Glass: How to Tell the Difference (5 Easy Tests)

You're holding a clear, glassy-looking stone and you're asking yourself the age-old question: is this actual quartz, or just a hunk of glass?

Don't feel bad. This is genuinely one of the hardest visual calls in casual rock collecting. Clear quartz and glass look really similar at a glance. They're both transparent, both shiny, both smooth. Even experienced collectors double-check sometimes.

But here's the thing: they're actually quite different once you know what to test for. Here are five quick, practical tests you can do right now with stuff you already have.

Close-up of someone doing a scratch test on a rock with a steel nail

Test 1: The Scratch Test (Most Reliable)

This is the gold standard, and it's dead simple.

Quartz has a hardness of 7 on the Mohs scale. Glass is about 5.5. That means quartz scratches glass easily, but glass cannot scratch quartz.

Grab a piece of glass you don't care about, like an old jar, a scrap piece, whatever. Try to scratch the glass with the sharp edge of your mystery stone.

Leaves a clear scratch on the glass? → Likely quartz.
Slides across without scratching? → Probably glass (or something softer than quartz).

Now flip it: try scratching your stone with the glass. If the glass scratches your specimen, it's not quartz. Real quartz is too hard.

Important: Make sure you're seeing an actual scratch and not just a powder residue. Wipe the surface and look again under good light.

Test 2: The Bubble Test (Look Inside)

Hold your specimen up to a bright light and look inside it carefully. A magnifying glass or your phone camera zoomed in helps a lot here.

Glass almost always contains tiny round air bubbles. They got trapped when the glass was molten. They're usually spherical and scattered throughout.

Quartz doesn't have round bubbles. It might have inclusions: wispy clouds, needle-like crystals (rutile), phantom layers, or small fractures. But these are irregularly shaped, not perfectly round.

If you see round bubbles? Glass. Case closed.

If you see weird internal structures that look like frozen clouds, needles, or rainbow fractures? Almost certainly natural quartz.

Test 3: The Temperature Test

This one's surprisingly effective and kind of fun.

Pick up your specimen and press it against your cheek or the inside of your wrist (somewhere sensitive to temperature). Then do the same with a piece of glass you know is glass.

Quartz feels cooler and stays cool longer. It's a better conductor of heat than glass, so it pulls warmth from your skin faster, creating that "cold stone" sensation. Even after holding quartz in your hand for a minute, it still feels notably cooler than glass would.

Glass warms up quickly to your body temperature and stays warm.

This test is subtle. It won't work well in extreme temperatures. But side by side, you can definitely feel the difference.

Test 4: The Weight Test

Quartz is denser than glass. Not by a huge margin, but enough to notice if you have a comparison piece.

Quartz has a specific gravity of about 2.65. Most glass is around 2.5 (though lead glass is heavier). If you hold a piece of quartz in one hand and a similarly-sized piece of glass in the other, the quartz should feel slightly heavier.

If you want to get precise, you can weigh your specimen and measure its volume using water displacement. Divide weight by volume and compare to 2.65. But honestly, the hand-feel comparison works fine for most situations.

Test 5: The Fracture Pattern

Look at the broken edges (if your specimen has any). Both quartz and glass break with conchoidal fracture, those smooth, curved, shell-like surfaces. So that alone doesn't help.

But here's the difference: natural quartz usually shows crystal faces somewhere, even partially. Look for flat, geometric surfaces alongside the curved fractures. Glass never has crystal faces because it's not crystalline. It's amorphous (atoms arranged randomly, not in a crystal lattice).

Also, natural quartz often has a slightly milky or cloudy base that transitions to clearer areas. Manufactured glass tends to be uniformly clear or uniformly colored.

Bonus: What About Sea Glass?

Beach finds make this even trickier. Sea glass (tumbled glass fragments) can look a lot like tumbled quartz. The same tests apply, but the scratch test is your most reliable friend here. Sea glass won't scratch a glass bottle. Tumbled quartz will.

Also, sea glass comes in colors that quartz rarely does: bright green (beer bottles), cobalt blue, or brown. If it's an unusual vivid color, it's probably glass.

Still Not Sure?

Sometimes a visual check is all you need. If you're still stuck, Rock Identifier can help. Snap a photo and the AI will analyze what you're looking at. But for most cases, the scratch test alone gives you a definitive answer in about five seconds.

The great thing about quartz is that once you learn to spot it, you'll see it everywhere. It's the second most abundant mineral in Earth's crust, and it comes in so many gorgeous varieties: amethyst, citrine, rose quartz, smoky quartz, chalcedony, agate. A whole world opens up once you can confidently say "yep, that's quartz."

Want to learn more about identifying rocks beyond quartz? Check out our beginner's guide to rock identification or see if your find is one of the 15 most common backyard rocks.