Hard translates the same word as that rendered “firmly cast” in verse 23b. Here the reference is to the fearlessness of Leviathan. Good News Translation retains stone as an adjective, “his stony heart is without fear.”

The poetic movement in line b is to a more specific kind of stone, the nether millstone, which refers to the lower of two circular stones in which the upper one is turned so that grain between the stones is ground to flour. The lower millstone was larger and harder than the upper stone which ground against it. For illustrations and descriptions see page 566.

Quoted with permission from Reyburn, Wiliam. A Handbook on Job. (UBS Helps for Translators). New York: UBS, 1992. For this and other handbooks for translators see here .