PASCAL GOETGHELUCK / SCIENCE PHOTO LIBRARY PASCAL GOETGHELUCK / SCIENCE PHOTO LIBRARY
Chalcedony. This red variety of chalcedony is known as carnelian, or sard. Chalcedony is a cryptocrystalline (minute crystals) variety of the mineral quartz (silicon oxide). This chalcedony has formed as nodules in volcanic rocks where cavities were filled by silica-rich solutions. It is a red colour due to traces of iron oxide that were deposited during its formation. This sample, from France, is 10 centimetres wide.
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