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Precious opal - The special characteristic of these gems is their opalescence, more appropriately referred to as "play-of-color". The more predominant types within this group are white opals that are translucent to semitransparent with play-of-color against a white bodycolor, black opals that are translucent to opaque with play-of-color against a black or other dark bodycolor, jelly opals that are colorless, transparent to semitransparent with little or no play-of-color, and boulder opals which are opaque and made up of a thin layer of opal occurring in ironstone matrix; the opal is cut and polished for the top and the matrix stone is left as a backing that adds strength and a dark background. Opal always contains water in variable amounts, but it can be as much as 30%. If not cared for properly, opals can lose water over time causing crazing, or cracking. White opals will be the most common found in jewelry and the value of an opal gemstone will be more determined by its play-of-color than by size.
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Fire opal - These opals are some of the most interesting since they do not look like what the average consumer would identify as an opal. They are transparent to semitransparent with or without play-of-color on a yellow, red or orange bodycolor. The most important deposits of fire opal are in Mexico and, for that reason, these beautiful gemstones are often called "Mexican Opals".
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Common opal - Very common, as the name would imply, and mostly opaque without play-of-color.
Opals, an optional birthstone for the month of October, are relatively soft gemstones and do not react well to heat or sudden changes in temperature. Opals are prized by many for their phenomenal play of color, and while they do make beautiful pieces of jewelry to be worn and appreciated, extreme care must be taken to protect these gems from damage. Because of their moisture content they require special care even when they are not being worn. When purchasing opal jewelry it is important to discuss their proper care with a gemologist, to insure that their beauty will be maintained and passed on from generation to generation.