In Revelation 4:3, what encircled the throne, gleaming like an emerald?

Answer: A rainbow

John saw a throne standing in heaven, and a rainbow that gleamed like an emerald encircled the throne.

“At once I was in the Spirit, and I saw a throne standing in heaven, with someone seated on it. The One seated there looked like jasper and carnelian, and a rainbow that gleamed like an emerald encircled the throne.”— Revelation 4:2-3 (BSB)