JEWEL BLOCKS, in the sea language, a name given to two small blocks which are suspended at the extremity of the main and fore top-sail yards, by means of an eye-bolt driven from without into the middle of the yard-arm, parallel to its axis. The use of these blocks is, to retain the upper part of the top-mast studding-fails beyond the skirts of the top-fails, so that each of those fails may have its full force of action, which would be diminished by the encroachment of the other over its surface. The haliards, by which those studding-fails are hoisted, are accordingly passed through the jewel-blocks; whence, communicating with a block on the top-mast head, they lead downwards to the top or decks, where they may be conveniently hoisted. See SAIL.