The word “ruby” comes from the Latin rubeus, meaning “red.” Before 1800, red gemstones, including red spinel and the red garnets, were referred to as ruby.

Ruby is corundum, the same mineral as sapphire, composed mainly of aluminum and oxygen. Pure corundum is colorless, with impurities of the element chromium causing the red color of ruby. A chromium content of about 1 percent is best for rubies, with excess chromium levels causing material to become merely an unattractive gray mineral not worthy of the name ruby.

The Mohs hardness of corundum is 9, second only to diamond at 10. Reddish corundum is called ruby, while all other colors are referred to as sapphire.

 

A Mogok ruby with a grade of 6-S is slightly dark and is considered the finest color, called “pigeon’s blood.” In spite of its dark color, this material shows a pronounced mosaic pattern because of its high level of transparency, and similar material may come from Thailand. In lighter colors, however, Mogok rubies become a blight, slightly purplish color, whereas Thai material is often a more dull, blackish color. This is due to a combination of factors-the presence of the blue-causing element titanium, whose influence becomes more visible as the red color becomes lighter, and Mogok ruby’s strong red fluorescence. Iron impurities present in Thai material, on the other hand, suppress its fluorescence and give it a blackish color.