Gold (chemical symbol: Au) is a chemical element with atomic number of 79, belonging to metallic elements. Gold, a simple substance, commonly known as "gold", is a popular precious metal, which has been used as currency, preservation and jewelry for many centuries. In nature, gold exists in gold nuggets or particles in rocks, underground veins and alluvium. Gold is also one of the currency metals. Gold is solid at room temperature, with high density, softness, brightness and corrosion resistance, and its ductility and ductility are the highest among known metals. The bright yellow of pure gold is traditionally considered attractive. Before the end of the Bretton Woods Agreement, gold was the cornerstone of the gold standard monetary system. The ISO currency code of gold bars is XAU.