Manganese tarnishes slowly in air and oxidizes ("rusts") like iron in water containing dissolved oxygen. Pure thulium metal has a bright, silvery luster, which tarnishes on exposure to air. Their lustre tarnishes rapidly in air due to oxidation. When freshly prepared, it has a silvery-white metallic lustre, but then slowly tarnishes in air. It is a hard, slightly malleable silvery metal that quickly tarnishes in air and moisture. Neptunium metal is silvery and tarnishes when exposed to air. It tarnishes to a yellowish or brownish color and gives a black streak. Silver tarnishes to form a thin surface layer of silver sulfide. Because silver tarnishes quickly, other shiny metals were substituted. No culpability tarnishes its colors or weighs upon its figures. He injects steroids, instead of insulin, to her and thereby tarnishes her reputation in society. On exposure to air, chalcopyrite tarnishes to a variety of oxides, hydroxides, and sulfates. Silver-plate is less expensive than gold, but requires more maintenance because it tarnishes easily. The suppression of free expression and of political opposition tarnishes the country's reputation abroad. Metallic technetium slowly tarnishes in moist air and, in powder form, burns in oxygen.