New Revised Standard Version
Over the heads of the living creatures there was something like a dome, shining like crystal, spread out above their heads.

New American Bible
Above the heads of the living creatures was a likeness of the firmament; it was awesome, stretching upwards like shining crystal over their heads.

Douay-Rheims Bible
And over the heads of the living creatures was the likeness of the firmament, the appearance of crystal terrible to behold, and stretched out over their heads above.

And over the heads of the living creatures was the likeness of the firmament, the appearance of crystal terrible to behold, and stretched out over their heads above.

the likeness

Ezekiel 1:26 And above the firmament that was over their heads, was the likeness of a throne, as the appearance of the sapphire stone, and upon the likeness of the throne, was the likeness of the appearance of a man above upon it.

Ezekiel 10:1 And I saw and behold in the firmament that was over the heads of the cherubims, there appeared over them as it were the sapphire stone, as the appearance of the likeness of a throne.

Exodus 24:10 And they saw the God of Israel: and under his feet as it were a work of sapphire stone, and as the heaven, when clear.

Job 37:22 Cold cometh out of the north, and to God praise with fear.

Revelation 4:3,6 And he that sat was to the sight like the jasper and the sardine stone. And there was a rainbow round about the throne, in sight like unto an emerald. . . .

Revelation 21:11 Having the glory of God, and the light thereof was like to a precious stone, as to the jasper stone even as crystal.

crystal