better late than never: some stores and retail dealers in precious metals use XRF- X ray florescence to determine the mix of alloy metals in a gold sample. The hand held ones start at 12K and go up to 100K for lab machines… the machines use X rays to bombard a metal sample and analyze the X ray spectrum that is floresced by the metal…cheaper models are less accurate but give decent reading if calibrated for only one specific purpose. If calibrated for precious metals, there will be a computer readout giving the percentages of gold, silver, copper and other base metals… they cannot be used for general purposes, and have to be re-calibrated for each specific purpose- eg., gemstones of one class, old house paint for lead, painting for metallic coloring agents in dating old masters art work as other examples…sensitivity depends of x ray flux put out by the machines…drawbacks include insensitivity to light elements up to silicon and aluminum, better sensitivity for transition metals, heavy metals where inner electron orbitals are crowded together and can be flipped more easily to a higher energy state before relaxing to ground state by emitting an x ray photon… This is pretty technical… but is the principle on how they work… since the machines are cost prohibitive to individuals, usually only bought by businesses and universities for academic purposes.