Snails are not the only animals to gnaw mineral substances for skeletal
material. Many animals (deer, rabbits, porcupines, etc.) gnaw bones for
their calcium phosphate; this is one of the reasons why bones do not last
long when exposed on the surface of the ground. In Kansas (USA), rabbits
sometimes gnaw exposed chalk (calcium carbonate). Rodents in particular
need great quantities of calcium in their diet, because their front teeth
grow continuously and are constantly being honed down with use. But this
behavior is not unknown in humans, either. In areas where the diet would
otherwise be deficient in calcium, as in one part of the Andes, people have
the habit of grinding limestone and salting their food with it. When asked
why they ingest this tasteless substance, they say it makes them feel
better.
 
To return to the snails: Why not let them decide what they want to gnaw?
Give them a choice of several calcium-rich materials (say, a seashell, an
eggshell, and a piece of limestone) and let them choose what suits them
best. Then tell us all about it!
 
Andrew K. Rindsberg
Geological Survey of Alabama