When gold jewellery is exposed to moist air for an extended period of time, the surface becomes dull. Although pure gold does not react with oxygen, gold alloys including copper and other elements are commonly used in jewellery. These elements react with oxygen and diminish the appearance of gold jewellery. When copper is exposed to moist air for an extended period of time, it develops a dull green coating. Copper hydroxide and copper carbonate make up the green substance. A layer of gold is removed by the goldsmith to remove this coating. As a result, its weight decreased slightly.