Revelation 4:3

and he that sat was to look upon like a jasper stone and a sardius: and there was a rainbow round about the throne, like an emerald to look upon.

There is little that can be known positively about these symbols. Note that, "There is here no description of the Divine Being, so as to point out any similitude, shape, or dimensions."[22] If there had been, people would probably have made idols of it and worshipped it. Regarding the stones here mentioned, we do not know exactly what they were, nor their color with any certainty.

The whole subject of the relation of precious stones named in the New Testament to those in the Old Testament, to those of classical antiquity, and of modern mineralogy is one of great obscurity.[23]

The jasper ... Phillips' New Testament translates this "diamond," and many accept this.

The sardius ... The New English Bible (1961) translates this carnelian, which Ladd identified as "a fiery red stone."[24] Some suppose that the rainbow encircled the throne horizontally and that it derived from the prismatic character of the rock crystal (the glassy sea on which the throne reposed)[25] But what kind of rainbow could be described as "like an emerald to look upon"? This teases the imagination beyond reality. At any rate, taking the above as a good guess, we have the diamond, the ruby and the rainbow like an emerald, which three colors are supposed to represent God's purity (the diamond), God's wrath in judgment (the ruby), and God's mercy (the rainbow like an emerald). At best, such interpretations are fanciful and rest upon inadequate foundations. It is true, of course, that the rainbow (Genesis 9:12ff) is indeed a symbol of God's mercy and of his covenant with Noah that the earth would not again be destroyed by a flood, and that seed time and harvest, day and night, etc., would thenceforth continue as long as the earth stands. The fact of a rainbow encircling the throne of God recalls this, but the description of it injects a new element. Our comment here embraces references to such things because of the usual emphasis given to them in current, and even ancient, writings.

While admitting that, "It is doubtful if any symbolical meaning is intended by the choice of these stones,"[26] Ladd went on to point out that they were in different positions on the high priest's breastplate (Exodus 28:17ff), and that they are numbered among the foundations of the new Jerusalem (Revelation 21:19ff).

[22] Adam Clarke, Commentary on the Whole Bible, Vol. VI (London: Carlton and Porter, 1829), p. 988.

[23] Isbon T. Beckwith, The Apocalypse of John (Grand Rapids, Michigan: Baker Book House, 1919), p. 497.

[24] George Eldon Ladd, A Commentary on the Revelation of John (Grand Rapids, Michigan: Wm. B. Eerdmans Publishing Company, 1972), p. 72.

[25] G. R. Beasley-Murray, op. cit., p. 113.

[26] George Eldon Ladd, op. cit., p. 73.