I would like to know what form of Carbon diamond will form when it is melted and boiled, whether it forms graphite, diamond or coal, or whether it just forms seperate carbon atoms in the air.

But if carbon is vaporized in the presence of about ¼ atmosphere of a non-reactive gas (normally helium), up to 5% of it will form spherical molecules of C_n, called fullerenes or "buckyballs."

Actually, "buckyball" should be used only for C₆₀ because it's short for buckminsterfullerene, the formal name of this particular all-carbon molecule. Other fullerenes are known; C₇₀ can be purchased from chemical supply houses.

If diamond were to melt at normal pressure (below about 20 atmospheres), it would solidify as graphite. Actually, it's impossible to melt diamond at normal pressures because it converts to graphite before it melts! This was shown by the rather wealthy chemist Humphry Davy in the early 19th Century. He put a diamond in an evacuated bell jar, then focused sunlight on it to heat it. The diamond changed to graphite before his eyes.

Later, Davy and Michael Faraday burned a diamond and proved that the only product was carbon dioxide, showing that diamonds were pure carbon.

For more information, see

• Diamond: the Molecule of the Month, which includes a phase diagram for carbon.
• Diamond, Buckyballs, and Graphite at the University of Wisconsin -- Madison.
• Another MadSci answer about buckyballs, which gives more references.
• The information about Davy and Faraday was taken from J.R. Partington's classic text, A Short History of Chemistry, which is still in print.