The Fire Opal of Mexico
In the world of colored gemstones it is color which matters most, and a number of gemstones with outstanding color are highly regarded despite having some fairly obvious shortcomings. Opal is a good example.
All of the opals have a variety of gemstone characteristics which are, let us say, less than ideal. These include softness, a tendency to dehydrate and crack, physical weakness and sensitivity to heat and chemicals. They are the most delicate gemstones commonly worn. But despite these shortcomings, they continue to be very popular because of their unique color.
The traditional precious opal, with its unique play of color, has been valued since Ancient Rome. During the Middle Ages, precious opal was regarded as especially lucky because it displayed the colors of many different gemstones. Not every culture has had the same view, however. A well-known Russian superstition associates precious opal with the evil eye.
Recently we have seen growing interest in one of the lesser known opals, the so-called fire opal. This is an opal known not for its play of color, but for its vivid body color, which ranges from white to yellow to orange to red.
Like all opals, the fire opal is amorphous hydrated silicon dioxide. "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.
Opal is typically opaque and is cut en cabochon. Fire opal is the only kind of opal that can be transparent to translucent, and you will often find the higher-grade material cut in facets. People seeing these gems for the first time are often surprised to find out that they are opals.
The color range of fire opal can be can quite wide, and some pieces display multiple colors in a single stone. The brilliant sunny yellows are highly regarded, but it is the more saturated orange and orange-red which is especially valuable.
Fire opal is associated especially 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, USA and British Columbia, Canada.
A new deposits of fire opal has recently been found in the northeastern section of Brazil. This area produces a golden variety of fire opal that can resemble the hessonite garnet or the hyacinth zircon. It also produces the more traditional colors of red, orange and yellow.
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 (5.5 to 6.5 on the Mohs scale), it is best suited for pendants, earrings and brooches, not rings.