The Hebrew word mutsaq means cast metal, or something poured out and set hard — as molten metal poured into a mold that cools into permanent form. As an adjective it describes something of great solidity, firmness, and hardness. In Job, it describes both the physical hardness of creation and the firmness of God's works.
Job 37:18 uses mutsaq to describe the skies as 'hard as a cast mirror,' glorifying God's creative power — what He makes is of lasting substance. Theologically, mutsaq reminds us that God's word and purposes are cast — firm and immovable (Isaiah 40:8), while a hardened human heart must be replaced by a heart of flesh (Ezekiel 36:26).