Agate, esteemed the least valuable of the precious stones, is a variety of quartz occurring usually as rounded nodules, known as geodes, or veins in trap rock and serpentine. Silica enters into its composition largely, and usually alumina and oxide of iron are present. The layers of chalcedony, carneliau, amethyst, common quartz, jasper, opal, and flint form bands of variegated colours, and these bands in the polished agate, by reason of their peculiar and distinctive arrangements, give to the several varieties their respective names, such as ribbon-agate, fortification - agate, zone-agate, star-agate, moss-agate, clouded-agate, etc.; also agates are named from the substance which forms the predominant layers, for example, jasper-agate, flint-agate, etc. The cutting and polishing of agates is an industry at Oberstein, in Oldenburg, Germany, and in Scotland also, where they are known as Scotch pebbles. Agate is used in finger-rings, for seals, beads, small handles, burnishers of many kinds, bearings in delicate mechauism, pivots, and for the knife-edges jof weighing machinery, for which and other purposes its hardness peculiarly fits it.