Carbon occurs in nature basically in three different modifications. In highly cristalline diamond, the carbon atom is tetrahedrally surrounded by 4 other carbon atoms due to sp3 hybridisation. In sp2-hybridised graphite, the carbon is present in numerous superimposed planar layers, with each carbon atom covalently bonded to 3 other atoms. The amorphous carbon (carbon black) consists of arbitrary mixtures of sp2- and sp3-hybridised atoms and has no extensive order. Graphite is used in pencil leads, for example.
The nanostructures of carbon that are known today are basically all based on the layered structure of graphite. Single graphene layers are the simplest form of nanocarbon with one dimension in the nanometer range. Cylindrically rolled graphene layers, known as nanotubes, have two dimensions in the nanoscale range, while fullerenes have all three dimensions in the nanometer range. Furthermore, starting from the graphite structure, the so-called carbon nanohorns and the layered nanofibers are known. Carbon nanodiamonds are derived from the crystal structure of diamond.