The metal case enclosing the works of a watch. They have been made mainly of gold, platinum or silver, but some were made of ormolu, gilded bronze, pinchbeck, or rock crystal, and recent examples of chrome.
Although usually circular, some are oval, triangular, hexagonal, octagonal, square or of fantasy shapes (known as a 'form watch'). Cases of an open-face watch cover only the back, with the dial exposed through a glass; the front cover of a closed-face watch is attached by a hinge which is usually released by pressure on a button. Sometimes there is a fixed back cover and also a hinged outer cover, and sometimes there is a hinged front cover protecting the glass ('hunter watch' or 'half hunter'). Some cases are ornately decorated, with enamel or engraving, some have decoration of cloisonné, champlevé, repoussé, niello, or piqué, and luxury examples are decorated with gemstones or are made from a gold coin.
Some cases, especially of a 'clock watch' or a 'repeater', are pierced. A 'pair-case watch' has two cases, the outer case being usually ornately decorated and the inner case (called the 'box') plain. The first portable watch is said to have been made soon after 1500 by Peter Henlein of Nuremberg, and within the century watch-cases were being decorated with gemstones and worn as articles of jewelry.
From: An Illustrated Dictionary of Jewelry, autor: Harold Newman, publishers: Thames and Hudson