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Types of Opal can be divided into 6 different categories including crystal opal, black opal, white opal, fire opal, boulder opal, and finally matrix opal. The first four in this list are all solid opal while matrix opal and boulder opal have the host rock included with the gemstone.  Boulder opals consist of a thin layer of opalized material that has formed in crevices of the native ironstone. The host stone is left in place on the back of the finished gemstone. 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 formed 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 treatment the host rock would appear tan and hide most of the play of color.

Black opal is solid opal that has a natural dark or black background tone, white opal has a light or white background tone, and crystal opal has a clear 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 many varities can be found around the world. Black opal is the most highly sought after 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.