O-ring
From BME Encyclopedia
O-rings are elastic rubber (silicone, latex, nitrile, neoprene, etc.) rings used to "border" beadless jewelry such as flesh tunnels and plugs. O-rings essentially provide a flare for unflared jewelry which prevents loss or excessive movement of the jewelry. The most common o-rings are black rubber, but are available in a variety of colors and materials. Some materials can cause an allergic reaction in the wearer; people who experience irritation from black o-rings often find clear silicone o-rings to be much more comfortable.
