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 boulder opal and matrix 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 host rock 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 weak 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 enhancement the host rock would appear tan and mask 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 comes with a red or orange background tone and is found in Mexico. Australia produces most of the high quality opal gemstones although many varities can be found worldwide. 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. Pictures of opals can be seen at Directorygemstones.org.