No one has ever seen a perfect crystal of graphite, with the naked eye
at least. Pyrolitic graphite is not perfect graphite, it is loaded with
defects and impuritys.
If you had a cubic chunk of a perfect graphite crystal it would be a
perfect optical reflector on two of it's faces and a perfectly black
optical absorber on the remaining 4 faces. the sides would be so dark
that even under intense light there would be no specular reflection and
no diffusion whatsoever. the outer faces would be featureless. It's
tensile strenght would be over 70Gpa in the x-y plane and less than
50Mpa in the z plane. It's hardness would still be less than soapstone.
It would be the wierdest looking substance ever.