Bede — on Rev 4:3 (Commentary on Revelation)

“And a rainbow was around the throne, in appearance like an emerald. The rainbow, which is made by the sun shining on clouds, and after the flood was first made a sign of propitiation, by the intercession of the saints whom the Lord illuminates, signifies the Church being fortified. They are well compared to the emerald stone of great greenness; for as they expect an imperishable inheritance with perfect faith, they also powerfully protect others by praying.”