This is the 9372nd most frequent Japanese word.
硬
"硬" means "hard" or "solid."
硬い is used here as an adjective to describe the physical hardness of the rock.
この岩は非常に硬い。
This rock is very hard.
硬い is employed metaphorically to imply rigidity or inflexibility in personality.
彼の性格はとても硬い。
His personality is very rigid.
硬水 uses 硬 within a compound noun indicating water with high mineral content.