Granite has a compressive strength of 172 lbs a cubic foot. This is it's ability to resist being compressed, like when a bullet hits it. At 172 lbf it will start to break. Anything less does little to no damage.
.50 BMG hits with around 13,310 lbf. Overcoming that Compressive strength 77 times over.
Stone, in 3.5 has a hardness of 8 and 15 Hp per inch of thickness. For a total of 180 hp per cubic foot. Multiply that by 77 then add the hardness.