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Diamond structure. Molecular model of diamond, a form of the element carbon (C). Carbon atoms are shown as spheres (black) linked by covalent bonds (grey). Each carbon atom is positioned at the centre of a tetrahedron, linked to four others at the corners. This repeating rigid structure does not allow rotation about the bonds, making diamond the hardest known naturally-occurring material. Diamond is used as a precious gemstone, and in industry to tip heavy-duty cutting and drilling equipment. (KEYSTONE/SCIENCE PHOTO LIBRARY/ANDREW LAMBERT PHOTOGRAPHY)