An Expanse - Ezekiel 1:22

Ezekiel 1:22 ESV 

Over the heads of the living creatures there was the likeness of an expanse, shining like awe-inspiring crystal, spread out above their heads. 

Revelation 4:6

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:

This is the expanse that separates the waters from the waters, heaven from earth. God is above and earth is below. 

Genesis 1:6-7

And God said, “Let there be an expanse in the midst of the waters, and let it separate the waters from the waters.” And God made the expanse and separated the waters that were under the expanse from the waters that were above the expanse. And it was so.