A.T. Robertson Commentary 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." — Revelation 4:3 (ASV)

To look upon (ορασε). Locative case of ορασις, old word (from οραω, to see) for appearance (in appearance) as in Eze 1:5,26.

Like a jasper stone (ομοιος ιασπιδ). Associative-instrumental case of ιασπις, old word (Persian), used for stones of different colors, one opaque like opal, one translucent (21:11,18f., possibly here, only N.T. examples), one a red or yellow stone (Isaiah 54:12). Some even take it for the diamond. Certainly not our cheap modern jasper.

A sardius (σαρδιω). Old word, in N.T. only here and 21:20. The carnelian or other red stone, derived from Sardis (Pliny).

Rainbow (ιρις). Old word, in N.T. only here and 10:1. From Eze 1:28.

An emerald (σμαραγδινω). Adjective (from σμαραγδος, Revelation 21:19), of emerald (supply λιθω), in associative instrumental case after ομοιος. John sees no form for God (Exodus 24:10), but only the brilliant flashing gems. "In the vision the flashing lustre of the ιασπις and the fiery red of the σαρδ are relieved by the halo (ιρις) of emerald which encircled the Throne" (Swete). A complete circle.