one’s heart is as firm as a rock; stone-hearted ❍ 子龙从我于患难，～，非富贵所能动摇也。(《三国演义》 360) He has followed me faithfully through all my misfortunes. His heart is firm as a rock.No riches or honours would move him.
心如铁石xīn rú tiě shí
形容意志非常坚定，就象铁块、石头一样坚硬。have a heart of stone, one’s heart is as firm as a rock, with a steel cold heart