Quartz vs Glass: How to Tell Them Apart

Property Comparison

The Definitive Tests

1. Scratch test. Quartz (Mohs hardness 7) scratches glass; glass cannot scratch quartz. Try scratching a piece of ordinary glass with an inconspicuous point of the specimen — if it bites in and leaves a permanent groove, glass is ruled out as the specimen's identity. Note the test damages whatever gets scratched, so pick your surfaces carefully. Even obsidian, a natural volcanic glass, is only hardness 5–5.5.
2. Specific gravity (hydrostatic weighing). Quartz always lands in a tight 2.63–2.65 band. Glass runs 2.30–4.20 depending on formulation. Read it in one direction: an SG outside 2.63–2.65 rules quartz out; an SG inside the band is consistent with quartz but does not confirm it alone, because some glass formulations land in the same range.
3. Loupe at 10x. Round gas bubbles and swirl marks indicate glass — quartz does not contain them. On shaped or faceted pieces, also check for mold marks on the back or girdle. This is the single most productive quick check on tumbled stones, beads, and mounted goods.
4. Polariscope or refractometer. Quartz is doubly refractive (uniaxial positive, birefringence 0.009) and shows the characteristic 'bull's eye' optic figure; glass is singly refractive because it is amorphous. On the refractometer, quartz reads 1.544–1.553 with two shadow edges; glass gives a single fixed reading. Because some glass can read near quartz values, the SR-vs-DR behavior matters more than the number itself.

Common Mistakes

• Judging by look and clarity. Water-worn quartz, calcite, and glass slag can look identical in a photo. Clarity, sparkle, and color prove nothing — the properties that actually separate them (hardness, density, crystal structure) don't photograph at all. Measure first.
• Treating one passing test as confirmation. Glass SG spans 2.30–4.20, and some glass reads an RI near quartz's range. A single number inside the quartz band is consistent with quartz, not proof. Stack the tests: scratch + SG + loupe agree quickly when the answer is quartz.
• Trusting the word "crystal." Decorative "crystal" is a trade term for glass. If a specimen is sold as quartz crystal, the scratch test and the loupe check for bubbles settle it in under a minute.
• Skipping the loupe on beads and tumbled stones. Rounded surfaces hide a lot, but round gas bubbles and swirl marks are visible at 10x even through a domed surface, and mold lines on beads are a giveaway.

Frequently Asked Questions