How old is chalk?

Answer 1

Chalk forms under relatively deep marine conditions from the gradual accumulation of minute calcite plates shed from microorganisms over millions of years. Therefore, chalk is very old indeed.

Answer 2

Chalk is a naturally occurring form of limestone made of calcium carbonate that can be millions of years old. However, the type of chalk most commonly used for writing on blackboards is typically manufactured and can vary in age depending on when it was produced.