Rottenstone is the soft, friable mixture of sand and clay resulting from the decomposition of siliceous limestones by the action of percolating carbonated water. It may be grey or red, and contains 82 to 86 per cent. of alumina, 4 to 13 per cent, of silica, up to 10 per cent. of carbon and up to 5 per cent. of iron. It should be very fine-grained, meagre to the touch, and almost capable of floating in water. It is obtained chiefly from Derbyshire and South .Wales, and is a cheap and efficient polishing material for glass, silver, brass, or.other soft metal.