Corundum Crystals & Chemistry
Sapphires, and their close cousins, rubies, are members of the corundum mineral species. In gemology, a “species” is a mineral that has a definite chemical formula and a specific three-dimensional structure. Corundum is an aluminum oxide (Al2O3), and has a regular crystalline structure formed by repeating patterns of arrangement at the atomic level. Crystalline minerals are classified into seven different crystal systems depending on the symmetry of their repeated atomic units. Corundum shares a trigonal crystal system with other minerals such as quartz, calcite, and tourmaline.
A “variety” is a sub-group of a mineral species. Corundum comes in a number of different varieties, many of which are not as rare or valuable as sapphire. “Emery” is a common variety of grey corundum and is used as an abrasive. Even old lawn chairs may become coated with a thin layer of corundum if their aluminum surfaces have oxidized.
Different varieties of corundum are distinguished by characteristics of color, transparency, internal features, and optical phenomena. The key feature that defines a ruby is its intense red color. Sapphires are another variety of corundum and they come in all colors except red. In essence, rubies are red sapphires.