Aqua regia is an extremely caustic mixture of one part nitric acid and three parts hydrochloric acid, which dissolves gold, platinum, and palladium, but not the other noble metals. It is used in the analysis of minerals and alloys, as an etchant, and as a powerful cleaning agent. Its name, which is Latin for 'royal water', derives from the fact that it dissolves the 'royal' metal gold.
Aqua regia preparation involves mixing strong acids, and their reaction produces heat and evolves poisonous vapors. It's important to follow safety protocols when making and using this solution.