Physically, commercial granites are hard, compact rocks with fine to coarse grains of metamorphic or igneous origin.

Hardness: 6 to 7 on moh's scale
Density: 2.6 to 2.8 kg/cm3
Compressive strength: 140 to 210 n/mm2
Modulus of rupture: 15 to 25 n/mm2
Water absorption: 0.1-0.6%
Average wear: less then 1%
Porosity: quite low
Weather impact: resistant

Chemically, they are igneous/metamorphic rocks composed of quartz, feldspar & ferromagnesian minerals like kriolite, chlorite, garnet, etc. A typical granite will have following chemical composition:
Silica (sio2): 70-75%
Al2o3: 10-15%
Cao+mgo: less then 0.5%
Feo + fe2o3: 2-4%
Alkalies: 4-6%
Tio2: less then 0.5%
Loss on ignition (loi): less then 0.5%