Assay is the analysis of precious metal or ore to determine its purity and value. There are several methods and a combination of these might be used to establish the purity of Gold Coins or Gold Bullion Bars. Stone assay is one of the most common methods and has been used since ancient times. Gold is drawn across the surface of a touchstone and the colour of the line that is left varies according to the purity of the metal. The stone may be treated with chemicals that only react when the gold is of a certain purity. Other methods used include X-Ray fluorescence and fire assay – which, although the most accurate, is normally reserved for larger lots.