Graphite : 3.09-3.23 g/mL
Diamond: 3.5-3.53 g/mL
The density of the two compounds is greatly affected on how the atoms are arranged in space. Density on the first place is the amount of a substance that can be placed in a specific volume or container.
Diamonds has more tight structure making it close packed. This means that the carbon atoms in the diamond are arranged carefully so that it is nearer to each other as compared with that of graphite.
On the other hand, graphite has a less ordered arrangement of carbon atoms making it loose packed.
Tightly packed or closed pack molecules tend to have higher density than the loose packed because atoms are more compact.
Hope this helps :)