Ruby And Sapphire

Mineral(s): Corundum var. Ruby, Sapphire
Locality: Ruby: Dattaw Hill, Mogok Township, Pyin-Oo-Lwin District, Myanmar (Burma)
           Sapphire: Ratnapura, Sabaragamuwa Province, Sri Lanka

Here are the two varieties of corundum that everyone knows and love. On the left is a 1.2 cm ruby crystal perched on a shapely white calcite rhomb, and on the right is a 1.7 cm sapphire crystal of classic tapering shape and various shades of blue.

Corundum has a hardness of 9, which is one level below that of a diamond. The color of a ruby is typically attributed to chromium (Cr) and the color of a sapphire is from both iron (Fe) and titanium (Ti). The amount of Cr will determine the ruby's color, but for sapphire, a phenomenon known as charge transfer will determine the sapphire's color using both Fe and Ti. Take a look here for a more detailed explanation on charge transfer, impurities, and more that can affect the color of a mineral.