Question

Why rocks are so hard?

Answer 1

The formation history of a rock will tell you how stable it will be at the surface. If a rock crystalizes from magma at or near the surface, and even deep down, it is likely to be very hard and difficult to break. Some rocks such as peridotite come from so deep that they are unstable at the surface due to the high pressures that olivine and pyroxene form in.

Answer 2

Hardness comes-down to how the atoms of the rock’s minerals are bound to each other and how they are arranged. And that comes-down to where the rocks formed and what atoms were around when they formed