Carbon forms a huge number of compounds with a large number of elements. It is the only atom that has a high combining capacity to form compounds and molecules with other atoms and elements. Some elements contain only carbon atoms in their structure. These are the pure forms of carbon. They are called allotrophs. They differ by the geometrical arrangement of the carbon atoms. All these materials are cent percent pure, but usually, they have tiny amounts of impurities. Graphite and Diamond are the main allotropes of carbon. Fullerenes and nanotubes are recently discovered allotropes.
The purest form of carbon is diamond. Graphite was first converted into diamond in the year 1955 in the presence of catalysts such as iron, cobalt, chromium, or manganese. Diamond is considered to be the purest form of carbon because the composition of the diamond is homogeneous and is consistent throughout.
Diamonds are purer than graphite because the carbon atoms in diamond are more tightly bound than in graphite as a result of which it is difficult for the impurities to penetrate the diamond lattice. A 100% pure diamond would be transparent while diamonds with impurities are often colored blue or yellow.