What is chalk, why is it used, and how is it made?
A good explanation can be found on the billiard chalk Wikipedia page. Chalk is used to provide friction between the cue tip and cue ball during contact, to prevent a miscue.
Here’s a magnified image of pool chalk from Chris Renfro:
Billiards “chalk” should not be confused with naturally occurring “chalk” (sea-formed calcium carbonate sedimentary rock) which has a very organic appearance under a microscope:
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