Diamonds aren't stable and will eventually, over billions of years, decompose from their cubic molecular structure to carbon's more stable form, graphite, which has a hexagonal molecular structure.

Oh, here's another good gemstone related one!

Amethyst and citrine are both quartz varieties, and if the color source happens to be from traces of iron in the crystal lattice, one can be turned into the other. Heating amethyst can make citrine, and irradiating citrine can turn it into amethyst. This is because the only actual difference between the two is the valiance level of a specific election in the iron atom giving the stone its color.