Diamond is the hardest of all substances. Graphite, in contrast, is a slippery, soft solid most familiar to us as the “lead” in pencils. Both materials, in spite of their very different physical properties, contain only carbon atoms. The two substances differ solely in the nature of the carbon–carbon bonds holding them together. Diamond consists of a rigid three-dimensional network of atoms, with each carbon bonded to four other carbons via orbitals. The carbon atoms in graphite, on the other hand,are hybridized,so each bonds to only three other carbon atoms. This trigonal planar arrangement causes the atoms in graphite to lie in flat, layered sheets that can shear off of neighboring sheets. You experience this when you write with a pencil: Sheets of carbon atoms shear off,leaving a thin trail of graphite. There is a third substance found in nature that contains only carbon atoms: buckminsterfullerene. Like graphite, buckminsterfullerene contains only hybridized carbons, but instead of forming planar sheets,the carbons in buckminsterfullerene form spherical structures.