Rock-soap,

Rock-soap, a mineral consisting of silica, alumina, peroxide of iron, and water, the silica nearly one-half, the alumina and the water sometimes nearly each one-fourth of the whole. It is earthy, easily broken, black or nearly so, very soft, and easily cut with a knife, is greasy to the touch, and adheres strongly to the tongue. It is valued by painters for crayons. It is found in Poland, Thuringia, and Bohemia, and occurs in basaltic-rocks in the Isle of Skye and Antrim, in the form of nodules of a greenish-gray or brown colour. It is only found massive.