The density of granites used in building ranges from about:
160 lbs/ft3 = 2560 kg/m3
up to
190 lbs/ft3 = 3040 kg/m3
Rough numbers, but close enough.
Light grey and red true granites are down in the 160-170 lbs/ft3 range, whilst dark green and dark grey can range up through 170-180 lbs/ft3 and black granites (not truly granites) are typically in the 180-190 lbs/ft3 range.
Rule of thumb ... use 190 lbs/ft3 if the dimensions are approximate and the type of granite unknown, or 170 lbs/ft3 if you know the dimensions and the type of granite is not a black or dark green or dark grey.
Hope this helps
david