'Harder' in the Bible

A brother offended is harder to win over than a fortified city,And contentions [separating families] are like the bars of a castle.

O Lord, do not Your eyes look for truth?You [have seen their faithless heart and] have stricken them,But they did not weaken;You have consumed them,But they refused to take correction or instruction.They have made their faces harder than rock;They have refused to repent and return to You.

I have made your forehead like emery (diamond), harder than flint. Do not be afraid of them or be dismayed before them, though they are a rebellious house.”

But by the [remarkable] grace of God I am what I am, and His grace toward me was not without effect. In fact, I worked harder than all of the apostles, though it was not I, but the grace of God [His unmerited favor and blessing which was] with me.