Translation
The one sitting on the throne had the appearance of Jasper
and Sardius,
and His throne was encircled by an emerald rainbow.
Paraphrase
The one exercising authority was obviously pure in character, one whose punishments are to be greatly feared;
yet His authority, [and
by implication, His exercise of that authority and the character that guides it] is held in its proper place by His mercy.
Jasper, Sardius and Emerald
Jasper, was a different stone than what we call Jasper. Though it was found in a variety of colors, it had a translucent, transparent, and sometimes shiny aspect to it, and those characteristics are the ones in view here, not its color. It was a symbol of God’s purity. Sardius, also called Sardine, is deep red and resembles blood – possibly for punishment/judgment, or vengeance on His enemies. Emerald is green and is thought to be a sign of mercy.
Since an emerald rainbow encircles the throne, it means that God’s authority, His punishment of the wicked and His pure character, are hemmed in, or held in place, by His mercy. His power to punish will not be abused and His demand for purity is balanced by mercy. This fits with the purpose of the rainbow as a sign of mercy, a promise given to Noah that God would not judge the earth by water again.