sedimentary

Chalk

Calcium Carbonate (CaCO3)

Hardness: 1-3 on Mohs scale (very soft, can be scratched by a fingernail); Color: White to light gray; Luster: Dull, earthy; Crystal structure: Trigonal/Microcrystalline; Cleavage: None (conchoidal to irregular fracture); Specific gravity: 2.3-2.6

Color: White to light gray
Luster: Dull, earthy

Identified March 30, 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 of calcareous algae called coccolithophores. These settle on the seabed, forming ooze that lithifies over millions of years (primarily during the Cretaceous Period).

Uses & applications

Used in agriculture to raise soil pH, in construction for lime and cement production, as a filler in paper, paints, and plastics, and traditionally as a drawing or writing material.

Geological facts

The White Cliffs of Dover in England are the most famous example of massive chalk deposits. The word 'Cretaceous' actually derives from the Latin word 'creta', which means chalk.

Field identification & locations

Identify in the field by its extreme softness, white streak, and strong effervescence (fizzing) when in contact with dilute hydrochloric acid. It feels powdery or 'chalky' to the touch and is porous.