Carbon atoms have four electrons with which they form bonds with other atoms. In the carbon phase known as diamond, all four of these electrons form bonds with different neighboring carbon atoms to form a rigid lattice. This form of carbon possesses extreme hardness sufficient to scratch glass. Moreover, diamond refracts or bends light so sharply that a ray entering a well-cut diamond is reflected internally, so that the diamond assumes a sparkling appearance.
The graphite phase differs from diamond in that it forms bonds with only three neighboring carbon atoms instead of four. This results in a less-rigid lattice consisting of parallel planes of six carbon rings linked together to form a sort of mesh. The resulting substance has a softer texture that crumbles when subjected to sufficient pressure. Graphite also conducts electricity far better than diamond. It also melts at a slightly lower temperature than diamond.
Fullerenes consist of a combination of five carbon rings and six carbon rings meshed together in such a way that they form a hollow sphere. Fullerene spheres differ from one another in the number of carbon atoms they contain. Buckminsterfullerene, the first to be discovered, contains 60 carbon atoms. Buckminsterfullerene becomes a superconductor at extremely low temperatures. When subjected to sufficient pressure, its hardness exceeds that of diamond.
Carbonado is a polycrystalline diamond, which means that it does not consist of a single crystal but many small crystals a few microns in diameter. The name "carbonado" and its alternate name "black diamond" come from its dark color. In contrast to ordinary diamond, carbonado possesses many tiny pores. Other similar polycrystalline diamonds are yakutite and framesite.
In contrast to the cubic crystals of ordinary diamonds, a type of diamond called Lonsdaleite has crystals characterized by a hexagonal arrangement of molecules. Lonsdaleite occurs in places where meteors have struck the earth, such as Tunguska in Russia and the Meteor Crater in Arizona. Meteor impact may cause the formation of Lonsdaleite.
Amorphous means "without form." Amorphous carbon lacks an orderly crystalline structure and therefore is without internal form. Soot and charcoal occur in the amorphous carbon phase. Amorphous carbon is a good conductor of electricity.