As many of you no doubt have wondered, I was curious how many things on earth I could actually crunch with my teeth. Tooth enamel is the hardest substance in the human body, leading one to question why humans do not have an armor-like hide composed of this amazing organic substance. At 5 mohs (the unit of measurement for hardness) Apatite (the major component of tooth enamel) would unfortunately be unable to crunch glass, titanium or uranium. Hardened steel and emerald are also out, so no eating certain jewelry. Amazingly, platinum, iron, copper, tin and lead all rank below Apatite in hardness. I would rank these materials as among the hardest things teeth could scratch or otherwise damage. That means that potentially, if you could digest these substances, and you wanted to eat them, your teeth could do the rest.