Harder than teeth.

The listing

Tooth enamel is about a 5 on the Mohs hardness, similar to apatite, which sounds just like "appetite." In fact, enamel is mostly made of hydroxyapatite - that's how apatite got it's name. Some toothpastes use hydroxyapatite, aka toothrock, but you still shouldn't eat it. If you stir it up with some phosphoric acid and slaked lime, you get something called tricalcium phosphate, aka the yumrock, and *that* is a food additive. Now we're sounding appetizing. Is yumrock safe to eat? Who knows? Some concerned scientists tested commercial samples of yumrock, turns out some of it *is* toothrock. Really, they should have seen this coming - apatite is known to lie about it's identity. That's how apatite got it's name - from apatáō, Greek for ‘to deceive.’ To summarize, you shouldn't eat toothrock. But you are.

Listing is for one hydroxyapatite-free chunk of milky quartz. Kind of shaped like a tooth.