Corundum

Sapphires and rubies are both varieties of corundum. Just a trace of chromium in corundum will create ruby red color. Sometimes, natural gems form in a non-continuous manner. For example, a fracture in the Earth may add a new element to the chemical mix of a developing gemstone crystal. If the fracture is re-sealed and the new chemical mix is just slightly different, the crystal may continue to grow with new layers distinguished by different colors. This is known as color zoning. If a little chromium enters the mix of a developing sapphire, you may get a color-zoned corundum crystal that's part-sapphire and part-ruby, like this red-and-purple specimen from Winza, Tanzania. Photos © Rob Lavinsky, www.iRocks.com. Used with permission.

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