CHALK

Chalk is a sedimentary rock of biochemical origin. It is soft, white and porous. It formed in the deep ocean far from land via the gradual accumulation of the calcite shells of coccolithophores. Layers of chalk are known that are hundreds of feet thick, indicating extremely long periods of accumulation.

Note that many products called chalk are not calcite. Gypsum is a common replacement (for example the "chalks" used on blackboards), as is talc. True chalk, however, is always composed of minute crystals of calcite.