Sedimentary Rock

Chalk

Calcareous Ooze (composed primarily of coccoliths)

Hardness: 1-2 on Mohs scale (very soft and crumbly). Color: White, off-white, light gray. Luster: Dull, earthy. Crystal Structure: Microcrystalline, rarely visible crystals. Chalk has a very fine-grained texture and is porous. It feels fine and powdery and will readily mark other surfaces.

Hardness: 1-2 on Mohs scale (very soft and crumbly)
Color: White, off-white, light gray
Luster: Dull, earthy

Identified October 25, 2025More sedimentary rock →

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Physical properties

Formation & geological history

Chalk is a fine-grained, porous limestone that forms from the accumulation of microscopic marine organisms called coccolithophores. These organisms create elaborate calcium carbonate shells (coccoliths) that settle to the seabed after death. Over millions of years, these deposits compact and lithify into chalk. It typically forms in shallow, clear marine waters and is associated with Cretaceous period deposits, which were a time of extensive chalk formation worldwide.

Uses & applications

Historically used as a writing tool (blackboards) and in art. In industry, it's used as a filler in paints, plastics, and rubber, as a mild abrasive in polishing compounds, and in agriculture to neutralize acidic soils (agricultural lime). It is also used in the manufacturing of cement and quicklime. Due to its softness, it is not used in construction as a primary building material but crushed for aggregate or as a component in cement.

Geological facts

The famous White Cliffs of Dover in England are composed primarily of chalk, formed during the Cretaceous period. The word 'chalk' comes from the Old English 'cealc', which is derived from the Latin 'calx' meaning lime. Chalk is known for its ability to absorb water due to its high porosity.

Field identification & locations

In the field, chalk is easily identified by its distinctive soft, powdery feel and its white to off-white color. It will readily rub off on hands and can be scratched with a fingernail. It typically reacts with dilute hydrochloric acid, producing effervescence, due to its calcium carbonate composition. Common locations include coastal cliffs and inland deposits in regions that were covered by ancient seas, particularly associated with Cretaceous age rocks in Europe (e.g., England, France) and parts of North America.

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