sedimentary

Chalk

Calcium Carbonate (CaCO3)

AI-generated identification · may be incorrect

Hardness: 1 on Mohs scale; Color: White to off-white/light grey; Luster: Dull or earthy; Structure: Microcrystalline/non-crystalline appearance; Cleavage: None (conchoidal fracture); Specific Gravity: 2.3-2.6

Hardness: 1 on Mohs scale
Color: White to off-white/light grey
Luster: Dull or earthy

Identified July 8, 2026More sedimentary →

Explore Chalk in the encyclopedia →

Identify your own rocks.

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

App Store Google Play

Physical properties

Formation & geological history

Formed in deep marine environments from the accumulation of microscopic skeletal remains (coccoliths) of plankton. Most deposits date to the Cretaceous period.

Uses & applications

Used in agriculture to raise soil pH, in the manufacture of cement, as a filler in paper and plastics, and historically for writing or drawing.

Geological facts

The famous White Cliffs of Dover are composed almost entirely of chalk. It is a highly porous form of limestone that can hold significant amounts of water or oil in underground reservoirs.

Field identification & locations

Identify by its extreme softness (can be scratched by a fingernail), white rub-off on hands, and violent effervescence when exposed to dilute hydrochloric acid. Commonly found in coastal cliff areas or inland quarries.