KAJ R. SVENSSON / SCIENCE PHOTO LIBRARY KAJ R. SVENSSON / SCIENCE PHOTO LIBRARY
Petrified wood. A collection of polished pebbles of petrified (fossilised) wood. Petrified wood is formed when dead wood becomes embedded in sediment and starts to decay. Over a long time the organic material is replaced by silica (silicon dioxide) carried by infiltrating water. The silica forms deposits of minerals like chalcedony or opal, often preserving the wood's fine structural detail, as has happened here. Photographed in central Washington State, USA.
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