'Harder' in the Bible

A brother offended is harder to be won than a strong city: and their contentions are like the bars of a castle.

O LORD, are not thine eyes upon the truth? thou hast stricken them, but they have not grieved; thou hast consumed them, but they have refused to receive correction: they have made their faces harder than a rock; they have refused to return.

As an adamant harder than flint have I made thy forehead: fear them not, neither be dismayed at their looks, though they be a rebellious house.