sedimentary

Chalk

Calcite (CaCO3)

Hardness: 1 on Mohs scale; Color: Pure white to light gray; Luster: Dull, earthy; Crystal structure: Trigonal (microscopic); Cleavage: None visible in hand specimen; Specific gravity: 2.2-2.3

Hardness: 1 on Mohs scale
Color: Pure white to light gray
Luster: Dull, earthy

Identified July 20, 2026More sedimentary →

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Physical properties

Hardness: 1 on Mohs scale; Color: Pure white to light gray; Luster: Dull, earthy; Crystal structure: Trigonal (microscopic); Cleavage: None visible in hand specimen; Specific gravity: 2.2-2.3

Formation & geological history

Formed in deep marine environments from the accumulation of microscopic skeletal remains of calcareous algae called coccolithophores. Most deposits date to the Cretaceous period.

Uses & applications

Used in the production of lime, cement, and fertilizer. Historically used as a writing material and in cosmetics or as a builder's filler.

Geological facts

The White Cliffs of Dover are the most famous natural chalk formation in the world. Despite its name, most modern 'blackboard chalk' is actually made from gypsum.

Field identification & locations

Identify by its soft, powdery texture that easily leaves a white mark on surfaces and its vigorous fizzing when in contact with dilute hydrochloric acid.