Where iron is present it acts as a damper on the red fluorescence, i.e. Siam rubies and emeralds from South Africa and India may be almost inert. Natural yellow sapphires fluoresce yellow, synthetic yellow ones are inert. Natural colourless fluoresce orange, synthetic colourless are inert. Yellow-green synthetic spinels fluoresce bright green. Blue synthetic spinels fluoresce red, most naturals do not.
Fluorspar fluoresces brightly.
Natural emerald is usually inert, most synthetic glows bright red. Diamond fluoresces in all colours, the fluorescence may be analyzed with a spectroscope in some cases for identification. When cooled with dry ice or liquid nitrogen some diamonds show fluorescence lines at 415.0 nm and 504.0 nm. Irradiated diamonds may show a line at 594.0 nm which is diagnostic. Diamonds that fluoresce bright blue show a yellow phosphorescence when held in cupped hands after the light is turned off - the only blue fluorescing gemstone to do so.