'Sea of Glass' in the Bible

Also in front of the throne there was something like a sea of glass as clear as crystal. Around the throne, at the middle of each side were four living creatures dotted with eyes in front and behind,

Then I saw something that looked like a sea of glass mixed with fire, and standing upon the sea of glass were those who had conquered the wild beast, his statue, and the number representing his name; and they had harps that God had given them.