Back to Top
The Nature of Sapphires

Corundum Crystals & Chemistry

natural untreated blue sapphire rough crystals corundum

Natural blue sapphire crystals

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.

No Comments on Corundum Crystals & Chemistry


Related Topics

Search the Site


The Nature of Sapphires

Sapphire Varieties

Judging Quality in Sapphires

Common Sapphire Treatments

Sapphires as Heirlooms

Sapphire Origins & Sources

Sapphire Lore


Gem Laboratories & Certifications

Jewelry Information

Articles & Essays

Related Information