History and Formation
A ruby is a precious gemstone, red in color, and made of corundum (aluminum oxide). The world's best rubies have historically come from the Mogok Valley in Burma (Myanmar). This has been true for at least a thousand years, and possibly much longer.
By law, the largest Mogok rubies were to be turned over to the king of Burma. Supposedly, some large stones were cut up so that they could be sold instead. The British took over upper Burma in 1886 and formed the Burma Ruby Mines Ltd. This company modernized the mining and distributed rubies worldwide, but it eventually had trouble containing costs. Also, synthetic rubies became easier to manufacture, which cut into sales. The company abandoned its operations in 1931. Mining reverted to the old methods.
Other sources of rubies impinge on the very definition of the gem. A ruby is pink to red. Any other color of gem-quality corundum is a sapphire, and outside of Burma, multiple hues are often mined and marketed.
Sri Lanka is an important source of rubies having a more subdued pink color. These gemstones plus sapphires were traded through India and thence to the Mediterranean. King Solomon reportedly presented gems from Sri Lanka to the Queen of Sheba, and an officer in Alexander the Great's army wrote of the island's gems. Arabs visited the island (Serendib in Tales of the Arabian Nights), as did Marco Polo. When the British assumed colonial control, they did not upgrade the gem mining as they did in Burma, leaving it in the hands of local Muslims.
There are numerous other sources of rubies. The list has grown greatly since World War II, including under the Greenland ice sheet.
Mining and Production
90% of the world's rubies come from Myanmar and are sold through neighboring Thailand. There and at other mining sites, gem-bearing rock is carefully crushed and sorted mechanically by density. The final sorting is by hand.
Unlike with diamonds, it is normal to treat rubies before cutting and polishing. Various treatments will improve the color, improve the transparency by healing inclusions, or repair fractures. Heating removes unwanted purple and blue tinges and makes the color more uniform. Fractures are filled by coating the ruby with carefully-chosen lead glass powder, which is then heated and melted into the gem. The gem surface becomes uniform, improving its transparency.
Properties and Uses
A ruby is a crystal of corundum, aluminum oxide, containing a small amount of chromium impurity. Other impurities, such as copper, iron, magnesium, or titanium, yield other colors or no color, and the gem is deemed to be a sapphire.
Precious gems are graded according to the 4 Cs: cut, clarity, color, and carat. The finest ruby will be a vivid dark red. In fact, the red of a ruby is more vivid than might be expected. The color of a gem is normally caused by what light it absorbs. Ruby corundum absorbs yellow and green light, leaving the red. However, in addition, the absorbed light is re-radiated as a red luminescence. As a result, the light reflected from the gem is not much dimmer than the light that fell on it, unlike most gemstones.
All rubies feature needle-like rutile inclusions (called "silk") that reduce the transparency. Heating will reduce or eliminate these, but their absence may indicate a synthetic gem. In occasional gems, the silk reflects an interesting pattern at certain angles, such as a star (right) or a cat's eye. In these cases, the gem may be more valuable than if it is transparent.
Corundum is hardness 9 on the Moh scale. Only diamond (at 10) is harder. Consequently, a ruby is unlikely to pick up scratches that diminish its clarity. Occasional rubies are used as bearings or as the tips of instrumental probes.
The first laser used a synthetic ruby crystal. In 1960, Theodore Maiman at Hughes Research Laboratories used a light-pumping apparatus to amplify the red light emitted by the ruby.
The primary use of rubies is, of course, in jewelry.