Fire agate

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Fire Agate (type of Chalcedony)
High Grade Slaughter Mountain Arizona Fire Agate Rough.jpg
Raw Fire Agate prior to refinement. geode
General
Category Oxide mineral
Formula
(repeating unit)
Silica (silicon dioxide, SiO2)
Identification
Formula mass 60 g / mol
Color Yellow to Red
Crystal system Trigonal, Monoclinic
Cleavage Absent
Fracture Uneven, splintery, conchoidal
Mohs scale hardness 6 - 7
Luster Waxy, vitreous, dull, greasy, silky
Diaphaneity Translucent
Specific gravity 2.59 - 2.61
References http://www.fireagate.us Fire Agate Resource Site

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 iridescenct 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.