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Rough untreated sapphires.  Image from http://gandhi-gemstones.blogspot.com.

I’m late in my post for Gem of the Week (by two weeks!) but I have been busy with shop stuff and posts are on their way.  Allons-y!

Blue sapphires are the traditional September birthstone, and I grouped rubies and sapphires as they are the same mineral, corundum, also known as aluminum oxide (Al2O3).  Rubies are classically the red stones (though one persons’ ruby may be another’s pink sapphire) while all other colors are sapphires.  You can find a sapphire in just about every color of Crayola crayon, with names like Ceylon, Kashmir, and Padparadscha.  There are also sapphires that change color under different types of light, similar to Alexandrite.  Asterism is a trait among some gems, including some sapphires and rubies, seen as stars that seem to magically move across the surface of the gem with the light.  It arises from light reflecting off of regularly-arranged titanium oxide impurities (or hematite impurities in black star sapphires), also known as the silk.

Corundum is heavy for its size and hard at 9 on the Mohs scale, meaning that diamond (Mohs 10) is usually employed to cut it.  Even then the tools take a beating and so gemcutters usually charge extra.  Unless specified as untreated, it’s safe to assume that a ruby or sapphire has been treated, usually by heating, to enhance color and remove the silk.  Synthetic rubies and sapphires have been made for over 100 years, and nearly every red or blue stone has seen its time as an imitation ruby or sapphire.

The ruby was considered the king of precious stones to the Hindus and the best rubies were thought to protect the wearer from any kind of harm or discord.  In Burma, where many of the finest rubies are found, warriors believed the stone inserted under the skin would similarly render them invulnerable.  In China, rubies were placed under the foundations of new houses to bring good fortune to the building.  Sapphires were associated with the divine in early Christianity, and thought to be an antidote against poisons, but at the same time it was favored by those practicing magic for its ability to influence spirits.