Q: How hard is granite?
A: Granite receives a 7 on the Moh's scale of mineral hardness.  A mineral with higher grade can scratch the ones below. Check the grades for the different materials:

10   Diamond
9     Corundum
8     Topaz
7     Granite 
6     Feldspar
5.5  Knife blade
5     Apatite
4     Fluorite
3     Marble (Calcite) 
2.5  Finger nail
2    Hard plastic
2    Gypsum
1     Talc