Asked by: Ron Pirani, Massachusetts, US
Tooth enamel is mostly hydroxyapatite, which is a mineral form of calcium phosphate. The apatite group of minerals scores a five on the Mohs hardness scale; which makes enamel the hardest biological material. Tooth enamel is harder than steel, but a lot more brittle. So you can’t scratch your enamel on metal cutlery but you can chip it by trying to open a beer bottle with your teeth.