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|Fire agate (type of chalcedony)|
Raw fire agate prior to refinement geode
|Silica (silicon dioxide, SiO2)|
|Formula mass||60 g / mol|
|Color||Yellow to red|
|Crystal system||Trigonal, monoclinic|
|Fracture||Uneven, splintery, conchoidal|
|Mohs scale hardness||6 - 7|
|Luster||Waxy, vitreous, dull, greasy, silky|
|Specific gravity||2.59 - 2.61|
Fire agate, a variety of chalcedony, is a semi-precious natural gemstone found only in certain areas of northern Mexico and the southwestern United States. Approximately 24-36 million years ago these areas were subjected to massive volcanic activity during the Tertiary Period. The fire agates were formed during this period of volcanism when hot water, saturated with silica and iron oxide, repeatedly filled cracks and bubbles in the surrounding rock.
Fire agates have beautiful iridescent rainbow colors, similar to opal, with a measurement of hardness on the Mohs scale of between 6.5 and 7 which prevent issues of fading, cracking and scratching. The vibrant iridescent rainbow colors found within fire agates, created by the Schiller effect as found in mother-of-pearl, is caused by the alternating silica and iron oxide layers which diffract and allow light to pass and form interference of colors known as fire. There is no actual object inside the stone, this special effect arises from light interference within the microstructure layering of the gem.