In the 1930’s, quite by chance, mineral prospectors noticed that certain rocks changed colour when exposed to filtered spectrum of a mercury lamp (a source of black light). During the Second World War the ultraviolet lamp gained importance. The U.S. Army realised that tungsten was of strategic importance for army equipment. However tungsten deposits had not yet been discoverer in the U.S.A.
Then large finds were made in the old mining regions. All the miners had to do was to switch on their ultraviolet lamps for the blueish-white colour of the tungsten ore (scheelite) to show. This is how the U.S.A. became the world’s major producer of tungsten during World War Two.
After the war, the search for uranium gave new impetus to the ultraviolet lamp. It was used by miners to differentiate between uranium and thorium, and in smaller mines to detect the zones richest in ore.
Some deposits were discovered purely by chance. A ranch owner in Nevada was going down into an old gold mine with his lamp when to his surprise the galleries lit up in a yellowish-green colour, the fluorescent colour of Autunite. A large deposit had been found. Thereafter, prospectors used ultraviolet lamps to search for deposits of lead, zinc and fluorite.