|Multicolor rough crystal opal from Coober Pedy, South Australia, expressing nearly every color of the visible spectrum. Credit: Dpulitzer|
Fire opal is a transparent to translucent opal, with warm body colors of yellow to orange to red. Although it does not usually show any play of color, occasionally a stone will exhibit bright green flashes.
Like other opals, fire opal is amorphous hydrated silicon dioxide by chemical composition. "Amorphous" indicates that opal has no crystalline structure; "hydrated" means that it contains water, typically from 3 to 10%. Opal is actually considered to be a kind of hardened jelly. But fire opal has some unusual characteristics not shared by other opals.
Fire opal glows with the fire of the sun: hot honey yellows, oranges, and reds so bright they look as though they might glow in the dark. Fire opal sometimes only occasionally exhibits an opalescence or play of color like other opals but it does not need this to take a starring role in jewelry. Its juicy color is just the right accent to earth tones or black and also looks great paired with other bright tones.
Fire opal is associated particularly with Mexico, and is mined in the Mexican states of Queretaro, Hidalgo, Guerrero, Michoacan, Julisio, Chihuahua and San Luis Potosi. The most important mines in Queretaro were discovered in 1835 and are still producing today. Small quantities of fire opal can also be found in Oregon in the USA and British Columbia in Canada.
Mexican opals are sometimes cut in their ryholitic host material if it is hard enough to allow cutting and polishing. This type of Mexican opal is referred to as a Cantera opal. Also, a type of opal from Mexico, referred to as Mexican water opal, is a colorless opal which exhibits either a bluish or golden internal sheen.
Unlike most opal, fire opal is sometimes faceted, so you can choose sparkle as well as color. Because it is light as well as bright, fire opal is especially good for earrings, where even small sizes have a big punch of color. Only the very finest qualities of Fire Opal are suitable for faceting.
Since fire opal has a high water content, it should be protected from heat and prolonged exposure to strong light, which could dry it out. Lapidaries cure fire opal by drying it before cutting to reduce instability, so it is actually not that delicate. But due to its relative softness, it is best suited for pendants, earrings, not everyday rings.