sedimentary

Chalky limestone

Micritic calcite (CaCO₃), likely biogenic limestone/chalk

AI-generated identification · may be incorrect

Soft, white to cream, dull and earthy with porous texture; calcite has Mohs hardness 3, rhombohedral crystals, and perfect rhombohedral cleavage, though crystals are microscopic here. It should fizz vigorously with dilute hydrochloric acid or strong vinegar.

Identified August 14, 2026More sedimentary →

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Physical properties

Formation & geological history

Forms from compacted microscopic marine shells and carbonate mud in shallow-marine environments, commonly during the Cretaceous and younger periods. The irregular fragment appears natural, though its exact age and locality cannot be determined from the image.

Uses & applications

Used for cement and lime production, soil neutralization, filler, and writing chalk; attractive pieces may be collected but have little jewelry value.

Geological facts

Its whiteness reflects high calcite content and low iron; chalk is a particularly fine-grained form of limestone made largely from coccoliths, fossil algae plates.

Field identification & locations

Common in chalk and limestone regions, including southern England, northern France, the U.S. Gulf Coast, and parts of the Middle East. Identify it by its very low hardness, powdery streak, light weight, and acid reaction; distinguish gypsum by the lack of strong acid fizz.