Quartz vs. Calcite: How to Tell Them Apart

1. Start With Hardness

Quartz is much harder than calcite.

Quartz — Mohs 7

It can scratch ordinary glass.

Calcite — Mohs 3

It is significantly softer and can often be scratched by a copper coin or steel point.

Note: for valuable, polished, or collectible specimens, avoid destructive testing unless you know what you are doing.

2. Look at the Crystal Shape

Quartz commonly forms elongated crystals with six-sided faces and pointed terminations.

Calcite can form many shapes, but rhombohedral crystals are especially common. Its faces often meet at slanted angles rather than forming the classic quartz point.

Crystal shape can be very helpful, but it should not be used as the only identification clue.

3. Examine How It Breaks

Quartz has no cleavage. When broken, it may show curved, shell-like surfaces known as conchoidal fracture.

Calcite has excellent cleavage in three directions. Broken calcite often produces smooth, repeated rhombohedral surfaces.

This difference can be one of the strongest clues when the specimen already has natural breaks.

4. Clear Calcite May Show Double Refraction

Some transparent calcite crystals can split light so that an object or printed line viewed through the crystal appears doubled.

This optical effect is called double refraction.

Clear quartz generally does not show this obvious doubling effect in the same way.

5. Acid Reaction

Calcite is calcium carbonate, so it reacts with dilute acid and may visibly fizz.

Quartz generally does not.

This is an educational distinction, not a recommendation for casual users to perform acid testing.

Common Mistakes