sedimentary

Chalk

Carbonate rock (mainly CaCO3)

AI-generated identification · may be incorrect

Hardness: 1-3 (Mohs scale); Color: White to light gray or buff; Luster: Dull/Earthy; Crystal Structure: Microcrystalline (Trigonal); Cleavage: None/Conchoidal; SG: 2.2-2.8

Hardness: 1-3 (Mohs scale)
Color: White to light gray or buff
Luster: Dull/Earthy

Identified February 28, 2026More sedimentary →

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Physical properties

Hardness: 1-3 (Mohs scale); Color: White to light gray or buff; Luster: Dull/Earthy; Crystal Structure: Microcrystalline (Trigonal); Cleavage: None/Conchoidal; SG: 2.2-2.8

Formation & geological history

Formed in deep marine environments from the accumulation of microscopic skeletons (coccoliths) of planktonic algae during the Cretaceous period.

Uses & applications

Used in agriculture to raise soil pH, in construction for cement manufacturing, as a writing material (blackboard chalk), and as a filler in paints and rubber.

Geological facts

The famous White Cliffs of Dover are the world's most iconic chalk formation. Most 'chalk' used in schools today is actually made from gypsum.

Field identification & locations

Identified by its soft, powdery texture and rapid effervescence (fizzing) when touched with dilute hydrochloric acid. It is very light and easily leaves a white streak.