Opal, the birthstone for October, is one of nature's most prized gems. The stone - which also happens to be the recommended jewelry gift for couples celebrating their 14th wedding anniversary - was mined by eastern Europeans, the Aztecs and the ancient tribes of Central Africa. Opals have been featured in the crown of the Holy Roman Emperor and the crown jewels of France. They were mentioned in the plays of Shakespeare and the novels of Sir Walter Scott. Napoleon gave an opal to Josephine. Queen Victoria gave them out as wedding gifts.

One of the reasons this gem has been so revered is because of its supposed mystical powers. Scandinavian women wore opals in their hair to prevent it from going gray. The Arabs thought opal would ward off lightning and grant invisibility to its wearer. Other powers ascribed to the gem include the ability to grant vigor, aid the heart and kidneys and protect against fainting and infection.

Worshipped by the Romans as a symbol of hope, fidelity, purity and good luck, opal is sometimes called the "queen of gems" because the stone can flash patterns of color representing every hue of the rainbow.

This "play of color" is one of opal's signature characteristics. The gem is found in a range of hues, including white opal (the most common); black opal; "boulder" opal (black opal with iron oxide); crystal or water opal, which is transparent; and fire opal, which has a yellow to orange to red body color.

The vast majority of the world's opal supply comes from Australia. Black opal is the rarest variety and therefore the most valuable. White opal is also mined in Brazil. Fire and crystal opal can be found in the United States (Nevada) and Mexico.

Brilliance of color and color pattern are critical in determining the value of opal. Opals with strong flashes of red fire are generally the most prized. Stones with blue or green flashes are more common and subsequently less valuable. Stone size also helps determine price, since the gem is very rare in larger sizes. Prices can vary from a few dollars per carat for common white opal to more than $1,000 per carat for fine black opal. Most stones are not faceted and usually cut into rounded cabochons to enhance color play.

Perfect natural opals are extremely rare and expensive. Many are treated to enhance their appearance. One common technique is to place the opal in a sugar solution and then in sulfuric acid, which blackens body color and makes the play of color more pronounced. Other treatments include applications of colorless oil, wax and resin, plastic, or synthetic resins and hardeners to fill cracks and improve durability.

With a hardness of approximately 5.5 on the Mohs scale, opal is relatively fragile, and care should be taken not to scratch, chip or crack it. To clean opal, use a soft cloth moistened with olive oil. Do not use chemical or mechanical cleaners. Also, avoid heat and dry conditions that could dehydrate and crack the stone. Avoid impacts.