sedimentary

Chalk

Calcium Carbonate (CaCO3)

Mohs hardness of 1, white color, earthy to dull luster, and fine-grained microcrystalline structure with specific gravity around 1.5 to 2.1.

Identified November 13, 2026More sedimentary →

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Physical properties

Mohs hardness of 1, white color, earthy to dull luster, and fine-grained microcrystalline structure with specific gravity around 1.5 to 2.1.

Formed from the accumulation of microscopic marine skeletal remains (coccoliths) in deep marine environments during the Cretaceous period.

Used in agriculture for soil conditioning, manufacturing lime and cement, construction, and traditional writing chalk.

Extensive chalk deposits form iconic white cliffs in coastal regions, representing millions of years of microscopic oceanic life.

Easily identified by its soft, powdery texture and vigorous effervescence when exposed to dilute hydrochloric acid.

Fossil-bearing concretion, possibly containing a fossil shell or other organic inclusion

Calcareous concretion; exact fossil and host-rock composition are not identifiable from the image

Sandstone or Siltstone

Arenite (if sandstone)

Fossiliferous Sedimentary Rock

Sedimentary limestone with invertebrate fossil imprint

Shale

Fissile Sedimentary Rock

Sandstone

Arenite

Shale

Clastic sedimentary rock