Manganese is an important alloying agent. It is added to improve the strength, toughness, stiffness, hardness, wear resistance, and hardenability of steels. Together with aluminum and antimony, especially in the presence of copper, it forms highly ferromagnetic alloys. Manganese dioxide is used as a depolarizer in dry cells and as a decolorizing agent for glass that has been colored green due to iron impurities. The dioxide is also used in drying black paints and in the preparation of oxygen and chlorine. Manganese colors glass an amethyst color and is the coloring agent in natural amethyst. The permanganate is used as an oxidizing agent and is useful for qualitative analysis and in medicine. Manganese is an important trace element in nutrition, although exposure to the element is toxic in higher quantities.