Cluster rings have a larger central stone, surrounded by smaller gems. They date back to the Georgian era but are as popular today as they were then. In fact, the ring that Prince William proposed to Katherine with (which once belonged to his mother) is a sapphire cluster ring.
Through the years they style has changed from a larger Georgian style, to a delicate Victorian cluster, through to the Art Deco geometric designs. Today the modern halo ring is the contemporary cluster ring, and many choose antique cluster rings as their engagement ring.