The Fire Opal of Mexico
In the world of colored gemstones it is color that matters most and a number of gemstones with outstanding color are highly regarded despite having some fairly obvious shortcomings. Opal is an example of this.
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 that are 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 Roman times. 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 there has been a 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 can be white to yellow, orange or red.
Like all opals, the fire opal is amorphous hydrous silicon dioxide. "Amorphous" indicates that opal has no definite crystal structure; "hydrous" means that it contains water; typically between 3 and 10 percent. Therefore, opal is considered to be a kind of hardened jelly. Fire opal has some unusual characteristics that are not shared by other opal varieties.
Opal is typically opaque and is usually cut en cabochon. Fire opal is the only kind of opal that can be transparent to translucent, and higher-grade material is often faceted. 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 quite wide, and some pieces display various colors in a single stone. Brilliant sunny yellow is highly regarded, but more saturated orange and orange-red colors are especially valuable.
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, USA and British Columbia, Canada.
A new deposit of fire opal has recently been found in the northeastern area of Brazil. This area produces a golden variety of fire opal that can resemble hessonite garnet or 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 direct light, which could dry it out. Lapidaries cure fire opal by drying it before cutting to reduce instability, so that it is not overly delicate. However, due to its relative softness (5.5 to 6.5 on the Mohs scale), it is best suited for pendants, earrings and brooches, rather than rings.
- First Published: March-04-2008
- Last Updated: August-22-2017
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