Minerology
Ruby and sapphire are the two varieties of the mineral corundum, that is, aluminum oxide (Al2O3). Red corundum is called ruby, and all other colors are called sapphire, but the distinction between ruby and pink or plum sapphire is so controversial that some jewelers may use Pantone color standards to distinguish them. (The name "ruby" is often a selling point for a particular gem.) They form hexagonal crystals and have a mineral hardness of 9, surpassed only by diamond in the natural world.
Large, gem-quality rubies have always been very rare. The huge gems described in medieval romances and oriental literature were most likely exaggerated by the imaginations the authors or were actually garnets or spinels.
History
The name ruby is derived from the Latin word for red, ruber. Ancient Orientals believed that rubies contained the spark of life, and that the ruby was self-luminous. Over the centuries, rubies have been regarded as symbols of freedom, charity, dignity and divine power. The Burmese believed that rubies ripened like fruit, and that a sapphire buried in the ground would eventually ripen into a ruby. A flawed ruby was considered "overripe".
In the Middle Ages, rubies were thought to bring good health and to guard against wicked thoughts, amorous desires and disputes. Their red color was said to cure bleeding. It was also believed that the ruby darkened in color to warn its owner of coming misfortunes, illness or death.
The first synthetic rubies were formed in 1902 by oxidizing aluminum powder in a hot gas flame, but the gems produced possessed gas bubbles and flecks of aluminum and were easy to identify. Today, it is possible to produce synthetic rubies that are indistinguishable from natural ones to all but the expert eye.
Today
Rubies are most frequently mined in Myanmar (known for clear, deep red "pigeon's-blood" rubies), Sri Lanka (medium-light red), Thailand (dark red to brownish-red), and across Africa (purplish-red). Heat treatment can be applied to improve color and eliminate imperfections, but this typically produces solidified borax within the gem which can be spotted under magnification. Because of their association with royalty, not to mention their rarity, large fine rubies are valued higher than a colorless diamond of the same size.
Ruby is the birthstone for the month of July and is the symbolic gemstone for the 40th wedding anniversary.