Hard rock is uncut by soft rock, and through the process of erosion.
Rocks come in a range of hardness values, mostly due to the differing strength of molecular bonds and crystal shapes. Deciding which rocks are hard really depends on what you wish to do with the rock.
A diamond is the hardest rock that we know of, but it is brittle and breaks easily. Some commonly known hard rocks are granite, gabbro, syenite, andesite, and of the latter, charnokite, gneiss, peridotite, slate, and unikite. “Hard” rocks and the minerals they contained or gave birth to, were the cutting edge of science before all the minerals were discovered.
