)
which is thermodynamically stable at pressures above
at room temperature and metastable at atmospheric pressure. At low pressures diamond converts rapidly to graphite at temperatures above
in an inert atmosphere. The chemical bonding between the carbon atoms is covalent with 
hybridization.
Note:
There is also a hexagonal diamond-like structure of the element carbon (lonsdaleite).
Source:
PAC, 1995, 67, 473
(Recommended terminology for the description of carbon as a solid (IUPAC Recommendations
1995))
on page 487