Revelation 4:6

And in front of the throne there seemed to be a sea of glass, resembling crystal. And midway between the throne and the Elders, and surrounding the throne, were four living creatures, full of eyes in front and behind.

Revelation 4:3

The appearance of Him who sat there was like jasper or sard; and encircling the throne was a rainbow, in appearance like an emerald.

Revelation 21:11

and bringing with it the glory of God. It shone with a radiance like that of a very precious stone--such as a jasper, bright and transparent.

Treasury of Scripture Knowledge

Summary

And the likeness of the firmament upon the heads of the living creature was as the colour of the terrible crystal, stretched forth over their heads above.

Bible References

The likeness

Revelation 4:3
The appearance of Him who sat there was like jasper or sard; and encircling the throne was a rainbow, in appearance like an emerald.
Revelation 21:11
and bringing with it the glory of God. It shone with a radiance like that of a very precious stone--such as a jasper, bright and transparent.