Spinel

Spinel is the stone most people have never heard of and the one collectors buy first. It is singly refractive, which means it holds colour with almost no extinction — a spinel looks lit from inside in a way sapphire rarely does.

It is also, unusually for a coloured stone, almost never treated. Spinel comes out of the ground the colour it stays. Sri Lanka produces vivid pink and purple, plus the rare cobalt blue that competes with sapphire on its own terms.