Calcite

One of the most common minerals on Earth and the main ingredient of limestone and marble. It breaks into perfect slanted boxes (rhombs) and fizzes in acid.

How to identify calcite

Fizzes in cold dilute hydrochloric acid (or slowly in vinegar).
Breaks into rhomb-shaped pieces along three cleavage planes.
Hardness 3 — a copper coin barely scratches it, a knife easily does.
Clear pieces show double images of text underneath.

Often confused with

Quartz
Quartz is far harder, has no cleavage and does not fizz.
Compare side by side
Halite
Halite tastes salty, breaks into cubes and dissolves in water.
Compare side by side
Dolomite
Dolomite barely fizzes unless powdered first.
Compare side by side

Where it's found

Iceland
Chihuahua, Mexico
Missouri-Kansas-Oklahoma Tri-State district

Uses

Cement, lime, antacid tablets, chalk and optical instruments.

Rock tumbling

Can be tumbled with care. Too soft for mixed loads — tumble alone with gentle grit.

Did you know?

Viking navigators may have used clear calcite 'sunstones' to find the sun on cloudy days.

Think you could spot it from a blurry close-up?

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