Three of the four named below are of one kind and the fourth is not. Choose the odd one out: Ruby, Emerald, Sapphire, Platinum.

The ruby, the emerald and the sapphire are precious stones: hard crystals dug out of rock, cut and polished so that light plays inside them. Platinum is a metal, dull grey until it is worked, and it is melted and drawn rather than cut. It keeps company with the other three in every jeweller's window, which is exactly what hides the split, but the tie among them is being a gemstone.