Perhaps no gemstone has been as prized throughout history as the ruby. Celebrated in the Bible and in ancient Sanskrit writings as the most precious of all gemstones, rubies have adorned emperors and kings and inspired countless legends and myths with their rich, fiery hues.

As the ultimate red gemstone, rubies have symbolized passion and romance for centuries. Ruby is the birthstone for July and is also the recommended gem for couples celebrating their 15th and 40th wedding anniversaries.

Also the color of blood, the stone is symbolic of courage and bravery. Warriors were said to have implanted rubies under their skin to bring them valor in battle and make them invincible. The stone has also been used as a talisman against danger, disaster, to stop bleeding, and a number of other ailments. Its intense color was thought to come from an undying flame inside the stone - or, as some legends would have it, a piece of the planet Mars.

Ruby is the red variety of corundum, a sister of sapphire. Like sapphire, ruby rates a "9" on the Mohs scale of hardness, making it the second hardest material known after diamonds.

The most valuable rubies come from Myanmar (formerly Burma), but they are mined throughout Southeast Asia. Good quality stones come from Thailand, Sri Lanka, and Vietnam. Kenya and Tanzania also are becoming more important as mining sources for ruby. But while the color of the stones from East Africa rivals the world's best rubies, most of these stones are fraught with inclusions that diminish their transparency and value. However, the East African stones are displayed to full advantage in cabochon cuts and have done well in the mass jewelry market. Meanwhile, Afghanistan, Pakistan and Russia have all produced occasional top-quality rubies, but the rough terrain in these areas has made mining difficult.

The most important factor to consider when buying a ruby is its color. It comes in a variety of shades ranging from purplish- and bluish-red to orange-red. Like sapphire, there is also a translucent variety of ruby that can display a six-point star when cut in a smooth domed cabochon cut.

The finest rubies are intensely saturated, pure red with no overtones of brown or blue. After color, the factors that influence value are clarity, cut and size. Rubies that are clear with no visible inclusions are more valuable than those with visible internal flaws.

Rubies are readily available in sizes up to 2 carats, and because of their intense color and durability, they make excellent accent stones. Larger sizes can be obtained, but top-quality rubies are rarer and more valuable than colorless diamonds - particularly in sizes above 5 carats. For instance, a 16-carat ruby sold at auction for $227,301 at Sotheby's in 1988. A 27.37-carat Burmese ruby ring sold for $4 million at Sotheby's in Geneva, Switzerland, in May 1995 - an astounding $146,145 per carat. In contrast, none of the D-color, internally flawless diamonds over 50 carats sold in the last decade can match this value per carat.

Rubies are rarely found perfect in nature - which is why many are heat-treated to intensify or lighten their color or improve their clarity. Heat enhancement is a permanent, stable process. Some rubies also have surface fractures and cavities that are filled with glass-like materials to improve their appearance. This filler may break, fall out or wear out over time if exposed to heat, strong abrasives or constant impact. For both treated or untreated stones, the safest cleaning method is to just use soapy water or a mild commercial solvent and a brush.