Mexian Fire Opal
The opal from Mexico plays a small, but signifigant part in the world opal market. The primary characteristic that sets it appart from Australian opal is it's yellow-orange-red base color. Vibrant flashes of fire emanating out of a rich reddish-orange crystal opal are very striking.
While this is the most prized base color, it is by no means the only base color found in the Mexican material. Mexican opal can have a water base, and degree of yellow to orange to deep red-brown, white, and very rarely seen, black. Rarely seen is a black treated stone called "smoked opal". The treatment uses hydrophane opals... opal that loses water content... which is soaked in honey water then heated to deposit carbon in the pores of the opal. The result can be quite attractive or somewhat rough. Smoked Mexican opal has a sticky feel when touched with the tounge or wet finger. This is because the opal attempts to absorb water.
Another type of Mexican opal is boulder opal, although it is not commonly refered to using that term. Opal is cut using the rhyolite as part of the stone. A dark appearance is usually caused by a black coating naturally occurring between the opal and rhyolite which provides contrast for the fire in the opal.
When pricing solid Mexican opals, it is necessary to distinguish three types There are clear orange crystal opals with play of color that are the best that Mexico has to offer. Then there are the translucent to opaque orange to white base stones with play of color. Finally, there are clear orange base color opals with no play of color. These stones are typically faceted rather than cabbed. While the industry calls these opals fire opal, this leads to great confusion as there is no play of color which is traditionally refered to as fire.
About text is from "Opal Identification and Value", Paul B. Downing, Ph.D.
J. Thomson Custom Jewelers