Revelation 4:6 (ESV)

6 and before the throne there was as it were a sea of glass, like crystal.And around the throne, on each side of the throne, are four living creatures, full of eyes in front and behind:

The brightness and majesty coming from God’s throne prompts John to lower his gaze so that he focuses on the floor beneath (or in front of) the throne. So pure is this floor that John needs a comparative to describe it; it is as it were a sea of glass, like crystal. The floor speaks of purity, of glory, of something radically not of this world, in no way touched by the finiteness, let alone the fallenness, characterizing this world (see also ?; ?, ?).