one’s heart is as firm as a rock; stone-hearted ? 子龍從我于患難，～，非富貴所能動搖也。(《三國演義》 360) He has followed me faithfully through all my misfortunes. His heart is firm as a rock.No riches or honours would move him.
心如鐵石xīn rú tiě shí
形容意志非常堅定，就象鐵塊、石頭一樣堅硬。have a heart of stone, one’s heart is as firm as a rock, with a steel cold heart