Did you know that rubies and sapphires are formed of the same mineral? They are both crystals of corundum (aluminum oxide), which is harder than any other naturally occurring mineral except diamond. So how does it happen that rubies and sapphires have such different colors?
Before I tell you that, let me also tell you that you almost certainly have a collection of corundum crystals in your home. I actually carry some in my purse everywhere I go and you probably do, too!
Corundum Is Hard
Because corundum is so hard, it makes an excellent abrasive. And the most common form of naturally occurring corundum is known as …. Drum roll, please… emery. Yes, emery, as in emery boards.
In the 1800s, the mineral emery (a mixture of corundum with magnetite, spinel and/or hematite) was crushed, glued to paper which was then glued to a stick to produce that vital manicuring tool, the emery board.
Emery boards today aren’t usually made of the crushed mineral emery, but they are still made of corundum, usually synthetic corundum crystals on the coarse side and garnet crystals on the fine side . The word “synthetic” here does not imply that the material is made of plastic or some other lesser material. The crystals are definitely aluminum oxide. “Synthetic” here only means that the corundum wasn’t used in the pure form as when pulled from the ground, it was instead purified and recrystallized in a lab.
Pure corundum is colorless. So how does it happen that rubies and sapphires have such different colors? The secret is impurities. If the aluminum oxide is contaminated with a tiny bit of chromium when it was crystallizing, it reflects red light. If contaminated with a tiny bit of titanium and iron, it will reflect rich, deep blue light.
Corundum crystals occur not only in shades of red and blue, but also in yellow, green, peach, orange and lavender. Red corundum crystals of gemstone quality are called rubies, while sapphire can refer to all of the other colors, including blue.
Corundum is not alone as a gemstone that derives its color from slight impurities. Garnet, aquamarine, emerald and topaz would all be colorless if not for impurities. Even the common humble mineral quartz becomes a gemstone (amethyst) when its crystals include a touch of iron.
Impurities. Perhaps it is fair to say that it is impurities that give our lives color.