Chalks may be divided into two categories:
natural and synthetic. Natural chalks consist of a pigmented material
in a clay base, which gives the chalk its distinctive soft texture.
The most common types are black, red and white, but it can occur
in a variety of colours. By the eighteenth century, over-mining
resulted in severe depletions, and for this reason (in addition
to the fact that natural chalks were uneven in quality) synthetic
chalks were developed.