Spinel is an attractive gemstones with fine red and pink varieties and interesting blues. Spinel is the magnesium aluminium member of the larger spinel group of minerals. It has the formula MgAl2O4 in the cubic crystal system. Spinel occurs in almost every color, and forms gems of all colors. Spinel is hard (8 on Mohs's scale) and has a singhe refractive index of 1.718 and specific gravity of 3.60.