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.