Denna loved mouthing rocks when she was younger, but she grew out of it by the time she was 6 months...
That was about when we started her on raw feeding. So, she gets lots of iron in her beef now... but decided her poop was tasty...
We tried DE (diatomaceous earth), and it seems to be helping!! Might work for a dirt/rock affinity, too? Worth a shot, anyway. Yuppie pet stores will carry "food grade DE" - and some human supplement stores might, too. We only give Denna 1/2+ Tablespoon with ~1cup of water in with her breakfast. I think that might be 25-50% of the recommended dose, but it seems to be doing the trick for us.