DIRK WIERSMA / SCIENCE PHOTO LIBRARY DIRK WIERSMA / SCIENCE PHOTO LIBRARY
Concentric banding in a thin, translucent slice of agate in a geode. Agate is chalcedony (amorphous or crypto-crystalline silica, SiO2), formed in voids and cracks by precipitation from liquids percolating in rocks, which are most usually of volcanic origin. The concentric banding of this example results from successive layers precipitating inside a rock void, proceeding from the outer rim towards the center and is known as a geode. The varied coloration result from different trace elements and other mineral impurities that were trapped as the agate formed. Agate is extensively worked into ordinary inexpensive jewellery, trinkets and other small ornaments. Actual height is 60 mm, provenance unknown.
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