‘And he who sat (on the throne) was to look on like a jasper stone (green) and a sardius (red), and there was a rainbow round about the throne, like an emerald (green) to look on.'
The rainbow ties in with Ezekiel 1:28, although there it is rainbow coloured. It may be seen as a reminder of God's covenant made to Noah (Genesis 9:13-17) and thus that God remembers His covenants made with the world and His people. The stones were among those depicted in the High Priests breastplate, but only as two among many, and the same applies to these stones as adorning the foundations of the New Jerusalem (Revelation 21:19-20). It would appear therefore that John's main aim is to depict what he saw in terms which described it physically. Compare ‘amber' in Ezekiel 1:27. John makes no attempt to depict the likeness of God. He avoids the descriptions in Ezekiel and Daniel. What he saw he considered to be indescribable.