DIRK WIERSMA / SCIENCE PHOTO LIBRARY DIRK WIERSMA / SCIENCE PHOTO LIBRARY
Polished agate, close-up. Agate is an aggregate of different forms of silica (SiO2) 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 bands (top) are variegated forms of chalcedony (amorphous or non-crystalline silica). Quartz (centre) is the crystalline form of silica. The colours and arrangements of microcrystals are influenced by changes in pressure, temperature, and mineral content that occur during the formation process. The horizontal layers at lower centre are the result of sedimentary deposition. Magnification: x1.8 when printed 10 centimetres high.
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