1 verse has
harder
in Edition Clear Qur'an by Talal Itani / Al-Israa (The Night Journey)

17:51Or some substance, which, in your minds, is even harder.” Then they will say, “Who will restore us?” Say, “The One who originated you the first time.” Then they will nod their heads at you, and say, “When will it be?” Say, “Perhaps it will be soon.”