Scientists may have found evidence of a lifeform that
eats gemstones -- specifically, a bacterium or fungus that can metabolize garnet.
Note that garnet is basically the #2 or #3 gem in hardness, right behind diamond, maybe sapphire depending, so these little buggers are chowing down on some real tooth-cracking energy sources -- and weirdly, they're probably metabolizing the iron (garnet is deep red due to iron content) in some kind of chemosynthetic process.
"Gem rot" was not a problem your mage expected to have, eh? Sorry about the jewel-eating fungus infestation.