Opal frequently adorns necklaces, earrings, bracelets and rings. It also appears in brooches and ornamental hair combs. Natural opals occur in many colors and are less expensive than many other gemstones. Synthetic versions are also popular and are difficult to distinguish from genuine stones.
Opals used in jewelry fall into one of three categories: black opal, white opal and fire opal. Black opals have a dark background illuminated by iridescent flashes of green, blue, purple and red. White opals have a milky, slightly translucent white-base color ornamented by brilliant sparks of color. Jewelry designers typically cut white and black opals into smooth cabochons (polished gems that aren't faceted) to emphasize their color-shifting properties. Fire opals, however, lack the extreme iridescence of other opals and benefit from faceting (cutting flat faces onto the gemstone). This emphasizes their rich red-orange hue and encourages light reflection, enhancing the stones' sparkle.
Soft, porous opals require careful handling and storage. Raw opals are vulnerable to dehydration and fracturing, a risk gem vendors minimize by storing the opals in water or in sealed plastic bags. Extreme heat dehydrates opals and permanently reduces their iridescence. Powerful acids and bases also destroy the stones.
Some gem vendors toughen their opals by impregnating them with clear plastic. This reduces the risk of dehydration and breakage but substantially reduces the opals' resale value. Another popular opal enhancement technique involves mounting white opals over dark backgrounds such as black glass or thin pieces of onyx. White opals mounted in this manner are difficult to distinguish from rarer, more expensive black opals.