Post Script
Upon moving to southern California, I searched for rocks that might make decent boiling stones. One of the locally-abundant rocks one can find is green (low iron content) and black (higher iron content) nephrite jade. While it is much harder than the talc/magnesite, the jade is quite carveable—at least with modern tools. The jade that is predominantly black never failed as a boiling stone.
The low-iron, green nephrite jade failed often, usually cracking and breaking after only one use. The talc/magnesite and the black nephrite jade performed perfectly, even after half-a-dozen uses!
Upon moving to southern California, I recently tried out some local rocks, like this talc. The tan/orange inclusions are magnesite, which is used to make furnace fire-bricks (and is one of the parent materials that metamorphoses into talc). I think that, while talc/magnesite also never failed to perform, it holds more heat than even basalt—my new favorite boiling stone!
