text: A metallurgical assay is a compositional analysis of an ore, metal, or alloy, usually performed in order to test for purity or quality.
  Some assay methods are suitable for raw materials; others are more appropriate for finished goods. Raw precious metals (bullion) are assayed by an assay office. Silver is assayed by titration, gold by cupellation and platinum by inductively coupled plasma optical emission spectrometry (ICP OES).
  Precious metal items of art or jewelry are frequently hallmarked (depending upon the requirements of the laws of either the place of manufacture or the place of import). Where required to be hallmarked, semi-finished precious metal items of art or jewelry pass through the official testing channels where they are analyzed or assayed for precious metal content. While different nations permit a variety of legally acceptable finenesses, the assayer is actually testing to determine that the fineness of the product conforms with the statement or claim of fineness that the maker has claimed (usually by stamping a number such as 750 for 18k gold) on the item.  In the past the assay was conducted by using the touchstone method but currently (most often) it is done using X-ray fluorescence (XRF). XRF is used  because this method is more  than the touchstone test. The most exact method of assay is known as fire assay or cupellation.  This method is better suited for the assay of bullion and gold stocks rather than works of art or jewelry because it is a completely destructive method.