Apart from precious opal, there is another type of opal known as common opal. It is usually fairly translucent. cloudy, and without opal's characteristic iridescence, due to the fact that the spheres of silica composing it are too large. It is generally rather a dull, pale, unattractive color; however, the best known and appreciated variety, fire opal. is brightly colored.
Appearance The color is yellow to orange, or brilliant scarlet. Fire opal may be a bit cloudy, or almost perfectly transparent. For this reason, it is both cut into cabochons and faceted, unlike precious opal. Transparent specimens may have good luster. Signs of iridescence are sometimes visible in bright light, the passage from common to noble opal being continuous, without a clear separation.
Distinctive features Fire opal is strongly characterized by its color, combined with an "amorphous" look, unlike that of transparent crystalline gems.
It has a very low density, lower than that of glass, with which it could be confused. On the other hand, it is quite hard, like other opals. There is no other stone that resembles it.
Occurrence It comes mainly from Mexico, but also from Guatemala, Honduras, and the United States. It is also found in some parts of Australia, which is, however, best known for the more precious varieties.
Value There is no comparison between the value of noble opal and that of fire opal, which is quite inexpensive even compared with other minor gems. It is worth some what more when a few splashes of color are visible inside "16 $i0"ﬁ‘- ll i5 Vﬂluﬂd by collectors as a curiosity but is little used in jewelry.
Simulants and synthetics It is hard to say whether or not certain types of glass of a similar color have been made to imitate this modestly priced gem. lt is not produced synthetically.