18k White Gold Spinel Ring QYCADJ .47cts, 2.02g total weight, Size 6.5 Spinel is composed of magnesium aluminate, and colored by chromium and iron. It is quite hard (8 on the Mohs scale), and it forms as cubic crystals like diamond. Spinel occurs in octahedral crystals, and has a complete absence of cleavage (unlike diamond). Due to spinel's excellent dispersion, spinel gemtones can possess vivid fire. The intensity of color is partly due to the fact that spinel is singly refractive. Most gemstones are doubly refractive. The best-known singly refractive gems are diamond, spinel and garnet.