How to Identify Calcite: Hardness, Streak, Luster and Tests

The properties that identify Calcite — mohs hardness: 3 (locally 2.5); streak: White; luster: Vitreous luster in crystals; massive form is dull; color: Transparent to translucent crystals are colorless, white, or light-colored; massive forms are often tan, gray, or white; cleavage or fracture: Three directions of perfect cleavage, producing rhombohedrons; specific gravity: 2.7. Hardness, streak and cleavage are the reliable ones; color alone rarely settles it.

• Dilute hydrochloric acid test: a drop of dilute HCl makes calcite fizz vigorously. Safety: Dilute hydrochloric acid is corrosive. Wear eye protection and gloves, avoid skin and eye contact, use only a few drops in a well-ventilated area, and rinse tools immediately afterward. Never use concentrated acid, and do not let minors handle it unsupervised.

• [Fluorite](/lookup/rocks-and-minerals/fluorite): Fluorite does not fizz in dilute HCl, while calcite does; fluorite (hardness 4) is slightly harder than calcite (hardness 3).
• Dolomite / Dolostone: Limestone (mostly calcite) fizzes immediately and vigorously in dilute HCl, while dolomite must be powdered before it will fizz.

Widespread from hot springs and underground caverns to coral reefs and seashells — found in limestone, marble, cave deposits, and veins.

Photograph a fresh, unweathered surface in daylight, then a second shot of any crystal faces or broken edge. Include a coin for scale. If you did a streak test, photograph the streak next to the specimen. The app suggests the likely mineral or rock; confirm with hardness and the tests above before you label it.