A ring-shaped coral island is called an atoll. According to Encyclopedia Britannica, an atoll is made up of sections of coral reef that form a closed shape around a central lagoon.
According to Britannica.com, the ribbons of reef that make up an atoll need not always be ring-shaped or circular, but they always form a closed shape around a lagoon of around 160 feet in depth. The coral reef is mostly underwater, with small islands or longer masses of land on the rim of the reef. Some atolls are inhabited. Inhabited atolls can be found in Polynesia, Micronesia and the Maldives.