Opals stand in a class by themselves. More than any other gem, each opal is distinctly an individual. No other stone has as rich and varied folklore. They are both one of the luckiest and unluckiest gems a person can own. They are so unique, they have their own descriptive vocabulary. Opals are also the most delicate gems commonly worn. They require special care to insure their health and longevity.
The name evolved from the Greek "Opallus" which means to see a change in color. Later, the Latin word "opalus" came to mean precious stone.
There is some doubt that ancient authors were referring to the same stone we call opal today. Some scholars believe that many references are actually to iridescent gems, like iris agate.
Opal’s fire was long thought to be the result of iridescence. However, with the advent of scanning electron microscopes, we now know that it is a result of diffraction.
 | Black opal, precious opal with black body color. Also used for black potch covered with thin layer of crystal opal that lets the black under layer show through. |
 | Semi-black, or gray opal, precious opal with dark body color. |
 | Light opal is in between semi-black and white. |
 | White opal, precious opal with white or very light body color. |
 | Boulder opal, a thin seam of precious opal on ironstone matrix. Since this is a natural occurrence, its value is higher than that of a man made doublet. The ironstone is very dark, which makes the fire stand out and gives a close resemblance to black opal. |