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 included. Boulder opals consist of a layer of opal that has formed in crevices of the host rock 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 weak on its own. Matrix opal is ironstone or clay stone in which the opal material has formed into the pores of the host rock. This type of opal is treated with a sugar solution and then acid to carbonize the sugar and create a dark background. Without this treatment the host rock appears brown and masks most of the play of color.
Black opal is solid opal that has a dark or black background 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 comes primarily from Mexico. Australia produces most of the high quality opal although various types can be found worldwide. 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. Pictures of opals can be seen at