Spread out above the heads of the living creatures was the likeness of an awesome expanse, gleaming like crystal.
KJV
And the likeness of the firmament upon the heads of the living creature [was] as the colour of the terrible crystal, stretched forth over their heads above.
ASV
And over the head of the living creature there was the likeness of a firmament, like the terrible crystal to look upon, stretched forth over their heads above.