The volcanic rock is occasionally spat out by the volcano in the Ettin Terrarium. They are initially red-hot, and can hurt your creatures if they touch them, but eventually cool down (or the hand can dunk them in water) to turn them into harmless (but confusing) toys. (They do not give the stimuli of having played with a toy when they are interacted with, leading to creatures learning that toys aren't fun.) Gnarlers use their long tongues to hurl the cooled rocks great distances to crack them into smaller parts, and gain nutrition from this activity, but the volcano does not erupt often enough to ensure a steady supply of volcanic rocks for the gnarlers to lick. See the gnarler article for hints on how to maintain their population.