Do you mean the type of chalk you would use to write on a blackboard? Or something different?
Calcium Carbonate (whiting powder) and French Chalk (talc) are the two substances commonly used by old school gunsmiths. I use them both still, but mainly as a degreaser and to soak up contamination when blacking barrels.
Calcium carbonate is/was used commonly as a degreaser when prepping the barrels and metal for blacking. It was mixed with either water or alcohol into a paste, applied with a rag, and then used to scrub the metal.
French Chalk was also used for the same purpose, depending on the location and its abundance.