In principle, carbon occurs naturally in three different allotropes. In the highly crystalline form known as diamond, each carbon atom is surrounded by four other carbon atoms through sp3 hybridisation. In the sp2-hybridised form known as graphite, the carbon sits in numerous planar layers stacked on top of one another, with each carbon atom covalently bonded to three other atoms. Amorphous carbon (carbon black) consists of any mixture of sp2- and sp3-hybridised atoms and has no long-range order. Graphite is used in pencil leads, for example.
Carbon-based nanomaterials have existed for a long time in nature. They are frequently formed during combustion processes, and small amounts are also contained in soot and ash. They are also released in volcanic eruptions. The fine craftsmanship of Arab swordsmiths led nanostructures of this kind to occur in Damascus blades, bestowing the blades with their unique strength and elasticity. Graphene layers scraped off from pencils have been used for marking and writing since the late Middle Ages. Despite this, the existence of nanocarbons was not proven until recently.
The carbon nanostructures known today are essentially derived from the layered structure of graphite. Individual graphite layers are the simplest form of nanocarbon, with one nanoscale dimension. Rolled-up graphene layers, so-called nanotubes, have two nanoscale dimensions, while in fullerenes all three dimensions are nanoscale.