|Rare Fire Agate
Agate is a form of chalcedony or microcrystalline quartz which forms in layers in a remarkable variety of colors and textures. The most familiar agate is the banded agate which has multicolored layers. Idar-Oberstein in Germany was once famous for its agate mines, and agate carving there has a long tradition which continues to this day.
Indeed agate has an even longer history, since the Egyptians used agate as a gemstone more than 3,000 years ago. But one of the rarest forms of agate -- fire agate -- has been available commercially only in the last 60 years.
Fire agate is a brown agate which has a botryoidal or grape-like growth form. What is special about fire agate is that it contains layers of plate-like crystals of iron oxide (limonite) in various planes. The iridescent colors of red, gold, green and, occasionally, blue-violet, result from interference between diffracted light rays traveling through and reflecting from these thin layers.
High quality fire agates can be as impressive in their play of color as fine black opal. But fire agate is much less expensive, even though it is significantly more durable than opal. Because fire agate is a form of quartz with a hardness of 7 on the Mohs scale, it is suitable for any kind of jewelry, including rings.
Fire agate is found in only a few locations in the world, mainly in the southwestern USA and Mexico. Deposits are found in the area between Kingman, Arizona and Needles, California, and around the Colorado River. Fire agate has also been found in quantity in some areas of Mexico.
Mexican fire agate specimens come from the mines of Calvillo in the state of Aguascalientes. There are dozens of mines around the mountains of Calvillo. Fire agates are also found in San Luis Potosi and Chihuahua, Mexico. Because of the mineral mixture, fire agate from Potosi only have golden fire, rainbow colors are extremely rare.
Cutting fire agate essentially reverses nature's process by grinding and polishing away layers, following natural contours, until only the fire is visible. It requires skilful work. Removing too much of the chalcedony reduces the iridescence, while removing too little results in a dull appearance. It is painstaking work and since few pieces can be cut in calibrated sizes, fire agate is rarely found in mass-produced jewelry.
- First Published: June-30-2008
- Last Updated: March-09-2011
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