DOUG MARTIN / SCIENCE PHOTO LIBRARY DOUG MARTIN / SCIENCE PHOTO LIBRARY
Obsidian. Lump of the volcanic rock obsidian. Obsidian is formed when volcanic lava is rapidly cooled on contact with water. The rock is mainly composed of silicon dioxide with impurities, such as iron, which give its black colour. Obsidian was used by Neolithic people to make tools because it leaves sharp edges when fractured.
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