Made chalk can be a item that you start with , thats how some people got theres is they re rolled toons till they got a few peices.Currently it takes chalk to make chalk and i dont think that is in gam eyet.
IRL chalk is found in the water of the NE section of Lake Tahoe. Apparently the water at the present time has a distinct bad taste and various Government groups are trying to solve the water problem. Mt Rose has big Chalk Area's and Shakespeare Peak has Chalk stains.
I didn't have to go far from my camp to find it, on the SE side of the lake near Round Hill. It will appear randomly and infrequently as a second in limestone deposits that are not directly near water, similarly to quartz and flint in granite.
I am still wondering if your perception or other stats or skills allow you to notice these "rares" more frequently.