Magnesium Aluminate, MgO.Al₂O₃

Natural spinel, or Magnesium Aluminate, MgO.Al₂O₃, crystals belong to the cubic system, have a density of 3.60-3.63 and a hardness of 8. Colourless, octahedral crystals of spinel are easily prepared by heating alumina and magnesia for a few minutes in an electric furnace. Their density at 15° C. is 3.57, and they are very resistant to ordinary chemical reagents.

Ebelmen obtained rose-coloured crystals by heating a mixture of alumina, magnesia, potassium chromate, and boric acid. He obtained other colours by adding suitable oxides.