The word(s) "hardness" appears 2 time(s) in 2 verse(s) in Quran in Abdullah Yusuf Ali translation.

(1) Thenceforth were your hearts hardened: They became like a rock and even worse in hardness. For among rocks there are some from which rivers gush forth; others there are which when split asunder send forth water; and others which sink for fear of Allah. And Allah is not unmindful of what ye do.

(2) Moses prayed: "Our Lord! Thou hast indeed bestowed on Pharaoh and his chiefs splendour and wealth in the life of the present, and so, Our Lord, they mislead (men) from Thy Path. Deface, our Lord, the features of their wealth, and send hardness to their hearts, so they will not believe until they see the grievous penalty."