GEOFF TOMPKINSON / SCIENCE PHOTO LIBRARY GEOFF TOMPKINSON / SCIENCE PHOTO LIBRARY
Polished blue agate. Close up surface of a polished slice of blue agate. The bands represent the edges of successive layers of deposition during the formation of the crystal. Agate is a type of chalcedony, which itself is a microcrystalline variety of the mineral quartz (silicon oxide). Agate is formed by precipitation from watery solutions within cavities in lava rocks. The presence of impurities can give a wide range of colours which can be regularly distributed in regular bands or zones, in clouds or in dendritic (branching) forms. This sample was found in Brazil.
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