Albert Barnes Commentary Ezekiel 1:22

"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." — Ezekiel 1:22 (ASV)

The color (Hebrew, eye) of the terrible crystal refers to the dazzling brightness of the firmament, a clear bright expanse between the throne and the living creatures, separating heaven from earth.