Treasury of Scripture Knowledge

Summary

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.

Bible References

The likeness

Revelation 4:3
And He who was seated [on it] appeared like jasper [i.e., possibly diamond. See 21:11] and carnelian [i.e., possibly a brilliant red stone], and there was a rainbow, resembling an emerald, surrounding the throne.
Revelation 21:11
It had the splendor of God [shining from it], and was as brilliant as a very precious stone, as crystal-clear as jasper.