And before the throne there was a sea of glass like unto crystal: and in the midst of the throne, and round about the throne, were four living creatures full of eyes in front and behind.
All Commentaries on Revelation 4:6 Go To Revelation 4
Bede
AD 735
glass: Because of our faith in the true baptism, this is compared to glass, in which nothing else appears on the outside than that which it has within. The grace of baptism is also represented by crystal, which is formed from water, congealed into a precious stone.