Mining, exclusively of industrial minerals for construction, played a minor role in Switzerland's economy. Metal mining has ceased, reserves of the small deposits of iron, nickel-cobalt, gold, and silver were mostly depleted, and new mining activities were discouraged for environmental reasons. Minerals produced in 2001 included hydraulic cement, common clay, gravel, gypsum, lime, nitrogen, salt, sand, stone, and sulfur (from petroleum refining). Metal processing—restricted to primary and secondary aluminum, secondary lead, and steel—depended on imported raw materials or scrap; environmental concerns have led to a policy to curtail or gradually cease smelting activities. The production of chemicals was Switzerland's second-leading industry and export commodity in 2002, and steel was the third top export commodity. A large diamond center, Switzerland was actively involved in cutting and polishing diamonds, and played a big role in international trade activities.