'Sea of Glass' in the Bible

and in front of the throne is what seemed to be a sea of glass, 'resembling crystal, while within the space before the throne and round the throne are four Creatures full of eyes' in front and behind.

Then I saw what appeared to be a sea of glass mixed with fire; and, standing by this sea of glass, holding the harps of God, I saw those who had come victorious out of the conflict with the Beast and its image and the number that formed its name.