buckminsterfullerene, graphite and diamonds may be pure carbon compounds but especially in the case of the former, obviously it's going to have a different density being a hollow spherical geodesic dome thing.
solid or liquid freebase *would* be consistent at a given temperature because it is the same stuff, the intermolecular forces are the same with any given sample of the identical pure compound at different temperatures and pressures.
The real point though, is that this, as a method of identifying a material, or the purity of a material, is far inferior to chromatography and mp/bp determination (of which i would be inclined to say the former is the simplest and cheapest and most accurate, especially tlc).