Job 41:16 Kabbalah

לִ֭בּוֹ יָצ֣וּק כְּמוֹ־אָ֑בֶן וְ֝יָצ֗וּק כְּפֶ֣לַח תַּחְתִּֽית׃

His heart is as firm as a stone; yea, firm as the nether millstone.

Read kabbalistic (mystical) interpretation of Job 41:16: and classical mystical sources.