Union road agate. Agate is an aggregate of different forms of silica (SiO2) that have formed concentric bands within a geode. A geode forms when the s

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Union road agate. Agate is an aggregate of different forms of silica (SiO2) that have formed concentric bands within a geode. A geode forms when the silica precipitates from liquid onto the inside surface of an empty pocket in a host rock. The outer layers are variegated forms of chalcedony (amorphous or non-crystalline silica), with the layers becoming more crystalline towards the centre. Union Road agates have been found in Lower Carboniferous limestone in eastern Missouri, USA. Many show fibrous structure on the outer bands.

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Photo credit: © NATURAL HISTORY MUSEUM, LONDON/SCIENCE PHOTO LIBRARY / Alamy / Afripics
License: Licensed
Model Released: No

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