These are two precious stones known for their dazzling beauty (when the light shines on them the right way). The point is not so much to compare God to the stones themselves as it is to capture in earthly words a little bit of the divine majesty of God. An earthly comparison, of course, never does complete justice to the heavenly reality.
3 And he who sat there had the appearance of jasper and carnelian, and around the throne was a rainbow that had the appearance of an emerald.