September’s Birthstone: Sapphire

Traditionally, blue sapphire is September’s birthstone, but since sapphire can be so many different colors, I say why not break from tradition and choose your favorite color?

Sapphire is a gemstone variety of the mineral corundum. Ruby is the red variety of corundum, and the term “sapphire” applies to any other color of gemstone corundum. Sapphires form in just about every color, including white, pink, orange, yellow, green, blue, purple, and black. Blue sapphire gets its color from trace amounts of the ions Fe2+ and Ti4+. Corundum is an aluminum oxide (Al2O3) mineral , and it forms in aluminum-rich, silica-poor geologic environments such as pegmatites (very coarse-grained granite), and contact zones between limestone and intrusive igneous rocks.

Economically important deposits of sapphire are found in Australia, Burma, Sri Lanka, and Thailand (Schumann, Gemstones of the World). Large sapphires are rare.

Corundum is one of the hardest gemstones, with a hardness of 9 on Moh’s hardness scale (where diamond is a 10). It is also a tough mineral (difficult to break).

Some sapphire exhibits an optical property called asterism. This type of sapphire is called star sapphire. It is sapphire with silk (tiny long mineral inclusions – usually rutile) growing in all directions, so that a 6-rayed star of light appears when viewed from certain angles. Star sapphire is best cut en cabochon. Because star sapphire is not worth as much on the market as traditional faceted sapphire, the gemstones are often heated to remove this beautiful optical effect.

Both heating and dying are common treatments applied to sapphire to increase its beauty and value.

Google image results for sapphire.
Sapphire on webmineral and mineral galleries.