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

The shine and color of these stones cause a special affection in the beholding, so that hereby the awesome and glorious shine of God is signified. The beams of the light cause a rainbow, as we see in the sun when it shines on a thick cloud through a thin cloud; or in a burning candle. Thus also in this sight a rainbow is caused by and showing the shine of him that sat upon the throne. This probably looks upon the rainbow of Noah, which is given as an everlasting token, not of the covenant of grace, but of the unchangeable promise that the world would not perish by water anymore. So that the unchangeable steadfastness of God in the executing of both all His decrees and of His promises, threats, and prophecies is signified by the rainbow here.

Not to be Ignored: Rev. Wilhelmus à Brakel’s Commentary on Revelation, Loc. 1707.