This gemological tool is helpful for identifying certain red, blue and green gemstones. It is used to determine whether a gemstone is natural or synthetic and can also be used for detecting Synthetic Blue Spinel as well as for detecting dyes in certain gemstones. The Chelsea Filter is a popular Gemological Dichromatic Filter that was originally devised by Anderson and Payne in 1934 at the Gem Testing Laboratory of the London Chamber of Commerce. The filter was devised with the collaboration of gemology students of the Chelsea College of Science and Technology. As this filter allowed transmission of both deep red wavelengths around 690 nm and yellow-green wavelengths, around 570 nm, that matched an Emerald's emission and absorption characteristics, it was initially recommended to assist the discrimination between natural emerald and its simulants such as Green Glass, Tourmaline, Peridot, etc. This discrimination was possible because Chromium - containing Iron - and Vanadium-Free Emeralds emitted a red fluorescence when illuminated by white light that had a content of ultraviolet wavelengths.
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