How are Rubies and Sapphires Synthesized?
Rubies and sapphires are different varieties of the mineral corundum, also known as aluminum oxide. Aluminum oxide is extremely common, making up more than 15% of the Earth’s crust, but it is usually impure, appearing as an opaque rock. When corundum is very pure, it is transparent, and is considered a gem. Red corundums are called rubies, while all other colors (most frequently blue) are called sapphire. Corundums are prized partly because of their extreme hardness — the only naturally occurring mineral of greater hardness is diamond. A ruby can scratch practically anything but a diamond. The synthetic production of ruby and other corundums began in 1837, when the chemist Gaudin made the first synthetic rubies by fusing chromium (pigment) with alumina at a high temperature in an environment containing oxygen. In 1847, Edelman synthesized white sapphire by fusing alumina in boric acid. In 1877, Frenic and Freil synthesized corundum crystals from which small stones could be cut. But it