sedimentary

Chalk (Sedimentary Pebble)

Calcium Carbonate (CaCO3) - Biogenic Limestone

Hardness: 1-3 on Mohs scale; Color: white to off-white/cream; Luster: dull, earthy; Crystal structure: Microcrystalline; Cleavage: none; Specific gravity: approx 2.0-2.8; Texture: fine-grained and porous.

Hardness: 1-3 on Mohs scale
Color: white to off-white/cream
Luster: dull, earthy

Identified April 11, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed in deep marine environments from the accumulation of microscopic skeletal remains of plankton (coccolithophores). Many deposits date back to the Cretaceous period (approx. 145-66 million years ago).

Uses & applications

Historically used for writing tools (blackboard chalk), cement manufacturing, specialized ceramics, leather tanning, and as a soil conditioner in agriculture to neutralize acidity.

Geological facts

The word 'Cretaceous' is derived from the Latin word 'creta,' which means chalk, reflecting the massive chalk deposits (like the White Cliffs of Dover) that formed during that geological era.

Field identification & locations

Identify in the field by its soft nature (often can be scratched by a fingernail), light weight, and the fact that it will leave a white streak on dark surfaces. It will also effervesce (fizz) when in contact with dilute hydrochloric acid.

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