An item generally round in shape and usually having a centrally located hole. It has at least one flat bearing surface. It is made of precious or semi-precious gem (ruby, sapphire and the like), natural or synthetic. It is intended for use in precision instruments, meters, timing devices, clocks, and the like.
mil-b-27497 mil spec all material responses overall material document and classification, 0.0625 inches nominal body outside diameter, 0.0600 inches nominal cup diameter, 0.0200 inches nominal overall width, 0.0157 inches nominal hole diameter