Why don’t we all turn to stone?

In Greek mythology, Medusa could turn people to stone by simply looking at them. Of course, we are all part stone: our bones contain a mineral called hydroxyapatite that makes them strong enough to bear our weight. This mineral is a form of calcium phosphate. It is highly insoluble, which is good, we don’t want our bones dissolving on us.

But this is also means that many of the fluids in our body have enough calcium and phosphate in them to form crystals of hydroxyapatite. But this does not happen, and we have only a poor understanding of why. We presumably have mechanisms to suppress nucleation and growth of hydroxyapatite crystals, everywhere except our bones and teeth, but what they are is unclear.

But anyway, I guess this is something to think about next time you see an ad on TV for healthy calcium for healthy bones. You already have enough calcium inside you to start to turn your tissues to stone.