Emery, the 'Armenian whetstone' of Theophrastus, is a variety of Corundum (q.v.), the mineral species which embraces ruby, sapphire, and other oriental precious stones, variously named according to their colour. The precious stones mentioned consist of nearly pure alumina in a crystalline condition, while emery is an amorphous admixture of alumina with varying proportions of oxide of iron and silica. Next to the diamond, these minerals are the hardest substances found in nature; but the hardness of emery is modified by the amount of iron and silica which enter into its composition. In external appearance, emery has nothing in common with the brilliant stones to which it is related, being a dense, opaque, dull, bluish-black substance, like a fine-grained iron ore. It occurs in large boulder-like masses on Naxos and the Greek Archipelago, near Ephesus, and at Westchester, New York. (Carborundum, a substitute, is made of carbon and silicon.)
It is prepared for use by first breaking it into lumps about the size of a hen's egg, then crushing these to powder by stampers. It is then sifted to various degrees of fineness, which are numbered according to the meshes of the sieve. Plate-glass manufacturers and others separate emery-powder into different degrees of fineness by the method of Elutriation (q.v.). Emery thus prepared is used for a great many important purposes in the arts. Being next in hardness to diamond-dust and crystalline corundum, the lapidary uses it for cutting and polishing many kinds of stone. Glass stoppers of all kinds are ground into their fittings with it. Plate-glass is ground flat by its means; it is also used in glass-cutting, and in grinding some kinds of metallic fittings. When employed for the polishing of metals, it has to be spread on some kind of surface to form a sort of fine file. Emery-paper, emery-cloth, emery-sticks, emery-calc, and emery-stone are various contrivances for such purposes. Emery-wheels are also largely used for smoothing and polishing the surface of iron castings, and in engineering work generally. These wheels consist principally of a mixture of emery-powder and hard vulcanised india-rubber. The polishing and buffing wheels employed by cutlers are also fed with emery-powder. See POLISHING.