An artistic representation of the sea of glass.
The Sea of Glass.
Revelation 4:6 "And before the throne there was was a sea of glass like unto crystal: and in the midst of the throne, and round about the throne, were four beasts full of eyes before and behind."
Location: Between the lower heavens (our planets atmosphere/outer space) and the third Heaven where God is.
Purpose: The sea of glass acts as a barrier between the holy, righteous, just, loving, kind, merciful, longsuffering, gracious God and the corrupt sin filled universe below. It should be noted that God did not like this division/separation as is clearly seen in Genesis 1:6-8 "And God said , Let there be a firmament in the midst of the waters and let it divide the waters from the waters.
And God made the firmament, and divided the waters which were under the firmament from the waters which were above the firmament: and it was so.
And God called the firmament Heaven. And the evening and the morning were the second day." As you can see he never called this firmament (the sea of glass) very good unlike other things he made during the recreation of the universe after Satan's(aka Lucifer's) rebellion.
Composition: The sea of glass is the water (from stars. Yes this is scientific go research stars producing water) that God flooded the universe with sometime in the distant past as judgement for Satan's rebellion. When God remade the universe (Genesis 1:2) he gathered these waters, placed them around the universe, and then froze them, creating an impassable barrier between the fallen sin filled universe and himself.