However, the masculine adjective in the Gk text, omoios (like) does not agree with the feminine gender of the word iris. While the word order seems to connect the comparison to emerald with the iris, the more important grammatical agreement seems to connect the emerald appearance with the One sitting on the throne. (The emerald likeness does not describe the throne, for though the adjective omoios agrees with thronou in masculine gender, it does not agree in case.) Though awkward, the verse could translate something like this: And He who sat there had the appearance of jasper stone and sardius — a radiance completely encircled the throne! — [He had] the appearance of an emerald. However, Oecumenius (6th century), who wrote the first extant Greek commentary on the Revelation, understood the emerald comparison as applying to the iris:
The natural rainbow, which the holy Scriptures call the “bow” of God, occurs from the reflection of the sun’s light, which when taken into the thickness of clouds is intercepted and produces multiple and various colors. But this spiritual rainbow that encircles the divine throne is of one color, for it was like an emerald, and this reveals the multitude of holy ministering angels, which surrounds God. And for this reason it is called a “rainbow,” even though it is of one color, in order that from the multitude of colors of the rainbow we might recognize the distinct orders of the holy angels. And yet, all are bound together into one color since all alike imitate their Lord according to his good works, and therefore the emerald color testifies to their sustaining work, even as jasper did for God. (Bold emphasis added. Source: Commentary on the Apocalypse 4.1-3, quoted in Ancient Christian Commentary: New Testament XII: Revelation, Intervarsity, 2005).Apparently Oecumenius was working from Greek manuscripts (of which there are many) that had omoia (feminine) rather than omoios (masculine). Interestingly, the Modern Greek Bible of 1850 employs omoia thereby making the emerald comparison explicitly apply to the iris, the rainbow. I’m inclined to agree and see omoia as the less problematic reading; until I find stronger contrary evidence, I will accept that the rainbow was in appearance like an emerald, i.e., a shimmering green.
Any biblical mention of a rainbow points us back to the first mention of the phenomenon after the flood of Noah (Gen 9.12-17). The rainbow symbolically speaks of God’s covenant-keeping and promise-keeping character (see Rev 10.1,6). In the Revelation the rainbow hints of mercy in the face of wrath (cf. Hab 3.2), and of a coming new earth, cleansed of evil, and even more glorious than the one repopulated by Noah’s family.