Rubies get their deep red color from chromium in the mineral corundum, which is also the hard foundation stone of sapphires. Rubies are rarer than sapphires, in part because sapphires encompass gemstones that range in color from blue to pink to orange. In contrast, rubies are always and only some variation of red, although jewelers have been known to market pink sapphires as rubies since rubies command a premium price.
While rubies are mined around the world, the biggest supplier of stones is Thailand. The corundum found there is plentiful, but it contains enough iron to give Thai rubies a brownish tinge. Just across the border, however, in Myanmar, iron is almost absent from that nation’s corundum, especially in the rock taken from mines in the Mogok district, source of the most desirable rubies in the world.
Though normally associated with high-end pieces of fine jewelry, small rubies have been used historically for more modest purposes. For example, in the Victorian Era, rubies were combined with other gemstones to create acrostic rings, in which the first letter in the name of each stone spelled out the word "dearest," (i.e., diamond, emerald, amethyst, ruby, emerald, sapphire, and topaz).
Still, rubies are a stone of royalty, the upper echelons of society, and movie stars. Mary Pickford, Paulette Goddard, and Merle Oberon are just a few of the leading ladies who wore ruby brooches, necklaces, bracelets, and rings (usually surrounded by diamonds, thank you very much). The Duchess of Windsor wore numerous ruby pieces produced by designers at Van Cleef & Arpels. Socialite Mona Bismark favored ruby-and-diamond necklaces, earrings, and bracelets by Cartier.
Like sapphires and many other gemstones, rubies are heat-treated to make their colors more vivid and rich. In addition to the Thai rubies that are prepared for market this way, rubies from Vietnam and Tanzania are also heat-treated to bring out the red in the stone. These rubies, including the ones mined near Songea in Tanzania, are often heated alongside beryllium to produce stones that are a beautiful reddish-orange. Unfortunately, beryllium is toxic, which means the people who work with it to produce finished rubies are probably doing so at their peril.
Also similar to sapphires, some rubies exhibit asterism, a star effect that can been seen when the stone is moved into and out of the light. In rubies, when fibers in a mineral called rutile line up just right, the result is a six-ray star.
Finally, one of the appeals of rubies is the fact that they tend to be imperfect and marked with any number of internal flaws. What’s interesting to the collector is that these cavities and mineral inclusions can often be used to pinpoint the ruby’s country of origin, as well as its manufacture. For example, rutile and calcite are common to rubies from Myanmar, while fracturing around inclusions within a stone can be evidence that the ruby was heat-treated.