Diamond is crystallized carbon, of density 35, and graphite is also crystallized carbon, but of density 22. . . . If a diamond is heated at ordinary atmospheric pressure to above 1,200C, then it turns black; it turns down to a graphitic powder. So the mere heating of graphite is of little help in trying to convert graphite to diamond. On the contrary, heat converts diamond to graphite.

—S. Tolansky, from The History and Use of Diamond, 1962.