VAUGHAN FLEMING / SCIENCE PHOTO LIBRARY VAUGHAN FLEMING / SCIENCE PHOTO LIBRARY
A cut and polished piece of bloodstone or heliotrope, a type of chalcedony. Chalcedony is a microcrystalline, variety of the mineral quartz (silicon oxide). The presence of impurities can give a wide range of colours, which may be distributed in regular bands or zones, in clouds or in dendritic forms. Bloodstone is an opaque green form with red markings, reminiscent of drops of blood. The red spots are caused by iron oxide impurities.
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