Spinel (Mg(AlO2)2)

The composition of natural spinel is MgAl2O4, crystal growth is much eased by growing Al2O3-rich material in the solid solution region. Colorless (pure), blue (Co), and other colored synthetic spinels made by the Verneuil process are widely seen in class rings and in other jewelry uses, where the blue is often mislabeled as synthetic sapphire.