en_tn_condensed/job/41/24.md

His heart is as hard as a stone

Possible meanings are: Leviathan's physical chest and the organs inside it are physically hard or Leviathan is not afraid of anything or Leviathan is spoken of as a person who kills without ever being sorry.

a lower millstone

"the hardest of rocks." The lower millstone is the larger and harder of the two rocks used to grind grain.