The rock mole is a rodent that can dig tunnels through the dungeon. They are able to eat anything metal, including amulets and some rings, as well as zorkmids. Sometimes they will even eat the iron spikes out of pits! Possibly somewhat surprisingly, they cannot actually eat rocks or stones (in vanilla), as no monster (currently) can.
When you find a rock mole in a treasure zoo, it is wise to kill it quickly before it eats all the gold.
You can remove punishment by getting a rock mole to eat your ball and chain.
Rock moles pick up gems they find buried in the walls. If you leave a rock mole alone for a while and then kill it, you can pick up the fruits of its mining. They will eat the gold pieces that also tend to be buried in walls, though.
A rock mole is a member of the rodent family. They get their
name from their ability to tunnel through rock in the same
fashion that a mole tunnels through earth. They are known to
eat anything they come across in their diggings, although it
is still unknown how they convert some of these things into
something of nutritional value.