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Sapphires 101

Corundum Crystals and Chemistry

Just What Are Sapphires?

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.

Rough sapphire crystals from kashmir
Rough sapphire crystals from kashmir

The Chemical Make Up Of Sapphires

Sapphires & rubies are corundum which is an aluminum oxide (Al2O3.).  Corundum has a regular crystalline structure formed by repeating patterns of arrangement at the atomic level.  Crystalline minerals are classified by seven different crystal systems that are separated depending on the symmetry of their repeated atomic units.  Corundum has a trigonal crystal system.

How Are Sapphires Categorized?

A “variety” is a sub-group of a mineral species.  Corundum (a mineral species) comes in a number of different varieties. Many of these varieties are not as rare or valuable as sapphire.  “Emery” is a common variety of corundum that is used as a commercial abrasive. Old lawn chairs may be 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.  Sapphires are a variety of corundum and comes in all colors, except this variety excludes red.  In essence, rubies are red sapphires because they belong to the same corundum species, just a different variety. Besides the incredible chemical makeup of sapphire, they are seriously strong stones, check out Sapphire Durability to understand what makes them so tough.