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Features
Fire agate is a gem made from concentric layers of colored silica and iron oxide, and before being cut, is also has a covering of chalcedony. The layers of silica and iron oxide allow light to pass through them in such a way as to display a metallic luster, called schiller. The appearance of this mineral is similar to opal.
In the gem world, a mineral's hardness is rated using the Moh's scale, a relative scale for testing hardness wherein a diamond receives a 10. On this scale fire agate receives a 7, meaning it is a hard gem.
Another important aspect of gems is how they break, which is called cleavage. Some gems break in symmetrical shapes, while others fragment, and these different ways are given labels. Other gems, such as fire agate, are labeled as having no cleavage, as they do not form a crystal lattice.
Composition
The chemical formula for fire agate is SiO2, meaning that it is made up of silicon and oxygen molecules. This chemical formula is shared by the whole quartz family. Metallic impurities in the silica result in the rainbow of colors in fire agate.
Formation
The most current understanding of how fire agate forms involves hydrothermal environments. Extremely hot water super-saturated with iron oxide and silica flows into subterranean spaces and is stopped, where it cools down. Then, chalcedony begins to form from iron oxide, lining the surrounding rock. Within this layer of chalcedony, concentric layers of iron oxide and silica form on top of each other. Limonite also forms, coating these inner layers. This coating of limonite causes the colors of the fire agate to flash like a diamond.
Sources
Fire agate has only been found in the southwestern United states and parts of Mexico. Within the U.S. fire agate has been discovered in the area between Needles, Calif. and Kingman, Ariz. It has also been found in the vicinity of the Colorado River. In Mexico, fire agate has been found in two states, Aguascalientes and San Luis Potosi.
Significance
Gem quality fire agate is rare. It is so scarce that the most sought-after gems, including ruby, emerald and diamond, are less rare by comparison.