Spinel is the magnesium aluminium member of the larger spinel group of minerals.common crystal forms are octahedra. Spinel is found in many colours and the most valuable is the burmeise red that can fetch in the US upwards of $6000 per carat for the right colour and quaility. Large gemstones are rare and also there is a star spinel that is very rare also. the refractive index is 1.712-1.762. It has a mohls hardness of 8. The british crown has a large spinal set in it that was thought to be a large ruby called the The Black Prince's Ruby is a bead-shaped spinel weighing roughly 170 carats and has been in the British Royal Family since 1400A.D