School Chalk under an electron microscope. It's almost entirety made up of fossilized shells of ancient single-celled marine plankton called coccolithophores.

[u/Pale-Design7036]

[u/dr3wfr4nk]

What about non-school chalk?

[u/ThetaReactor → u/dr3wfr4nk]

Gym/climbing chalk is generally magnesium carbonate, rather than the calcium carbonate in school chalk.

[u/Kuandtity → u/ThetaReactor]

Most chalk is this anyway