There are basically six distinct types of natural opal including black opal, crystal opal, white opal, fire opal, matrix opals, and boulder opal. The first four in this list are all solid opal while matrix opal and boulder opal have the host rock as part of the gemstone.  Boulder opals consist of a layer of opal that has formed in crevices of the native ironstone. When the stone is cut the host rock is left on the backside of the cabochon. This provides a dark background to the thin layer of solid opal. This is also left on for strength, as the thin layer of opal would be too fragile on its own. Matrix opal is ironstone or clay stone where the opal material has seeped into the pores of the host rock. This opal type is boiled with a sugar solution and then acid to carbonize the sugar creating a dark background. Without this enhancement the host rock would appear brown and mask most of the play of color.

Black opal is distinguished by a natural dark body tone, white opal has a light or white background tone, and crystal opal has a transparent background tone. Fire opal has an orange or red body tone and is found in Mexico. Australia produces most of the high quality opal gemstones although various types can be found around the world. Black opal is the most expensive type of opal followed by boulder opal, crystal opal, and fire opal. Matrix opal and white opal are the cheapest types of opal but still can reach hundreds of dollars per carat in their finest forms. Opal pictures can be seen at Directorygemstones.org.

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