Opal Types
Precious opal, which displays "opalescence" (spectral color, iridescence that changes with the angle at which the gem is viewed), is the kind of opal with which most people are familiar. There are three major kinds:1. White Opal - White opal is an opaque stone in which the colors appear as flashes or speckles.
2. Black Opal - Black opal contains fire with a dark body color. These are less common and tend to be costly. These are the investment high quality opals !
3. Opalized Organic Material - Opalized organic material may consist of wood, plant stem, bone or shell that has been petrified under extremely rare conditions.
Crystal Opal
Crystal opal, the next most costly type of opal, is transparent with flashes of color. It is highly valued for the brilliance of its colors and the fact that many layers of color can be seen within the stone.
Boulder Opal
Another unusual type of opal is boulder opal, which has opal with an ironstone host rock matrix which creates a natural dark background to view its fire. These sometimes occur in "splits," a matched pair of opals created when a price of boulder opal is split along the opal vein. These are particularly favored for earrings, since they are mirror images of each other.
Fire Opal
Fire opal is transparent or translucent with an orange or red body color. Fire opals are named for their reddish color but are opalescent. Be careful not to misinterpret the term "fire" as iridescence, because in precious opal (with a play of iridescent color) the play itself is also called fire. Much of the world's fire opal is mined in Mexico.
Common Opal
Common Opal is rather opaque with no spectral play of color. Many names are used to describe common varieties. Among these are honey opal, milk opal, and moss opal. Examples include milky agates and certain "petrified" opals.