The specific gravity of graphite is 2.3, which makes it lighter than diamonds with specific gravity of 3.52.
Also in terms of density:
Diamond has 4 carbon atoms bonded to each other with no gaps. Graphite, however, is 6 carbon atoms bonded covalently in a hexagonal structure, with the hexagons attracted by van der Waals so are held in layers. Therefore more air in graphite makes it less dense than diamond.
Density is inversely proportional to mass.
