New Revised Standard Version
Its heart is as hard as stone, as hard as the lower millstone.

Contemporary English Version
and its heart is stone.

New American Bible
His heart is cast as hard as stone; cast as the lower millstone.

Douay-Rheims Bible
His heart shall be as hard as a stone, and as firm as a smith's anvil.

His heart shall be as hard as a stone, and as firm as a smith's anvil.

as hard

Isaiah 48:4 For I knew that thou art stubborn, and thy neck is as an iron sinew, and thy forehead as brass.

Jeremiah 5:3 O Lord, thy eyes are upon truth: thou hast struck them, and they have not grieved: thou hast bruised them, and they have refused to receive correction: they have made their faces harder than the rock, and they have refused to return.

Zechariah 7:12 And they made their heart as the adamant stone, lest they should hear the law, and the words which the Lord of hosts sent in his spirit by the hand of the former prophets: so a great indignation came from Lord of hosts.