Fire Opal science
Many people mistakenly think that a fire opal is simply an opal with a lot of red showing in its beautiful, iridescent display of colors. This is an understandable mistake as the word “fire” usually refers to this beautiful iridescent display of colors opals possess, not the opal’s actual body color. A Mexican fire opal is actually very different than the opal we are used to seeing. While an opal is opaque and white, displaying an iridescent play of colors in every color of the rainbow, the Mexican fire opal is transparent to translucent, with a uniform red, orange, yellow, or brownish body color. Fire opals can come either with or without a play of color or fire. Iron content is what gives Mexican fire opals their orange color. The best, most highly prized specimen are transparent, have a play of color within the stone, and are of a fire-red orange color. These specimens can demand a retail price of up to $300 per carat. In contrast, translucent stones that are brownish or yellowish, sell for $5 a carat. Fire opals are usually faceted, and the wearer of the gem must be aware of the fragility of the stone. Like all opals, Mexican fire opals can crack in the course of time if it is not properly taken care of.
Mexican fire opals are named for their uniform flaming orange or cherry red body color. They are always very brightly colored, and can be a little bit cloudy to almost perfectly transparent. Transparent specimens have a good luster. Like the traditional opal, fire opals can occasionally display signs of iridescence in very bright light. Fire opals have a very low density, lower than that of glass, with which it is sometimes confused. Fire opals, like other opals, are relatively hard, rating a 5.5 to 6.5 on the hardness scale.