Why igneous rocks are hard?
Because the hardness of minerals in igneous rocks tends to be quite high.
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.