Description | The red crystals are spinel and the white matrix is calcite. Spinel, MgAl2O4 (magnesium aluminum oxide), is one of the few relatively common minerals of the cubic system. It typically forms octahedral (8 sided) crystals; the crystal in the middle of the sample is a good example. Spinel is sometimes prized as a gem. Spinel can have just about any color, but red is most common. It has occasionally been confused with ruby, most notably in crown jewel collections, but ruby is much more valuable and harder. |