en_tn/job/41/24.md

His heart is as hard as a stone

A rock or stone does not change and become soft. Possible meanings are 1) Leviathan's physical chest and the organs inside it are physically hard or 2) Leviathan is not afraid of anything or 3) Leviathan is spoken of as a person who kills without ever being sorry. (See: rc://en/ta/man/translate/figs-simile and rc://en/ta/man/translate/figs-personification)

a lower millstone

"the hardest of rocks." The lower millstone is the larger and harder of the two rocks used to grind grain. It would be the hardest rock people could find.