Why igneous rocks are hard?
Igneous rocks, like all rocks, consist of various kinds of minerals. Hardness in minerals is a function of the strength of their chemical bonds. In geology, the Mohs hardness classification was developed as a semi-quantitive way of the determining the relative hardness of minerals.
Diamond is the hardest mineral (
Hornblende (5-6) and Pyroxene minerals (5-6) are the main minerals in mafic basalts and gabbros.
So, the minerals in igneous rocks have a moderately high hardness and so igneous rocks themselves tend to be quite hard. They get even hard if they become a metamorphic rock.