sedimentary

Chalk

Calcium carbonate (CaCO3)

Hardness: 1-3 (Mohs scale), Color: White to light gray/yellow, Luster: Dull or earthy, Crystal structure: Microcrystalline, Cleavage: None (conchoidal fracture), Specific gravity: 2.2-2.8

Identified June 22, 2026More sedimentary →

Identify your own rocks.

Get a report just like this from any photo, free.

App Store Google Play

Physical properties

Hardness: 1-3 (Mohs scale), Color: White to light gray/yellow, Luster: Dull or earthy, Crystal structure: Microcrystalline, Cleavage: None (conchoidal fracture), Specific gravity: 2.2-2.8

Formation & geological history

Formed in deep marine conditions from the gradual accumulation of the microscopic shells (coccoliths) of planktonic algae over millions of years, primarily during the Cretaceous period.

Uses & applications

Used in the production of lime, cement, and concrete; as a building material; in agriculture to treat acidic soils; and traditionally for blackboard writing and gymnastics grip.

Geological facts

The famous White Cliffs of Dover in England are made entirely of chalk. It is a highly porous rock that can act as a massive natural filter and aquifer for groundwater.

Field identification & locations

Identify by its soft, powdery texture and white color; it will effervesce (fizz) vigorously when in contact with dilute hydrochloric acid. It is frequently found in coastal cliffs or rolling 'downs' landscapes.