Referring to an absolute, demanding, or ultimate challenge or measure of quality or capability – deriving from very old times, several hundreds of years ago – when nitric acid was used to determine the purity or presence of gold, especially when gold was currency before coinage. Gold does not dissolve in nitric acid, whereas less costly silver and base metals do. The use of nitric acid also featured strongly in alchemy, the ancient ‘science’ of (attempting) converting base metals into gold.
