The Lord had told Moses that no man could see God and live (Exodus 33:20). John can do no better than use comparative language (“had the appearance of”) precisely because he could not see the fullness of God himself (let alone find words to describe God). What John sees of God reminds him of jasper and carnelian.
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.