Carbon is available in nature as graphite and generally not as diamond. Artificial graphite is made by the reaction of coke with silica (SiO2).
SiO2 + 3C (2500°C)------> "SiC"----> Si (g) + C(graphite)
Artificial diamonds are made by the application of heat and pressure (> 125 kBar) in the presence of catalyst like fe, cr or pt. The metal melts on the carbon surface, the graphite dissolves in metal film, and the less soluble diamond precipitates out. The presence of nitrogen as an impurity gives yellow diamonds while boron gives bluish colours.
buckminsterfullerene is formed in the treatment of graphite by lasers.