DIRK WIERSMA / SCIENCE PHOTO LIBRARY DIRK WIERSMA / SCIENCE PHOTO LIBRARY
Obsidian rock. Obsidian is an igneous rock formed by the rapid cooling of volcanic magma or lava which contains granite. Obsidian has a glass-like structure because it contains high levels of silica, the basic component of glass. The rock's black colour is due to the presence of small amounts of iron. Once used by Stone Age humans to make tools, obsidian is now used industrially as raw material for rock wool. Photographed in Iceland.
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