His heart is firm as a stone. Some regard this as intended physically, and note that the great saurians, with their cold and sluggish circulation, have hearts which are comparatively torpid, not contracting or expanding readily.
Others take the "stony heart" to mean a fierce and obstinate disposition. In either case, the description will well suit the crocodile. Yea, as hard as a piece of the nether millstone. A repetition and slight exaggeration of the preceding idea.