Simple Fish Hook Clasp on a Pearl Necklace. Photo Courtesy of Lang Antiques.

Any fastener used in jewelry. There are many types of clasps from the very basic hook and loop to mechanical clasps that are spring loaded. They may be strictly utilitarian or they can be decorated with gemstones, enamel and engraving to the point that they can also serve as a focal point for a piece of jewelry.

Ruby and Diamond Clasp with Safety Latch. Photo Courtesy of Lang Antiques.