What is "chalks"

Usage examples of "chalks".

She had further confounded him by proving to him that although he could, with concentration, tell the chalks themselves apart most of the time, he literally could not tell a chalked line of green from one of blue in even the strongest artificial light—and it was not possible to take the finished diagram out into the sun.

She had a copy of the same diagram with her and had it lying open beside her bundle of colored chalks, but it helped to have someone outside it to see that she hadn't somehow overlooked something.

Subtle colors—the pale colors of the chalks, for instance—all looked very much alike to him.