The Chelsea Filter is one of the most useful gemological tools you will find...and one of the least taught in many gemological schools in the United States. This filter was developed by two British gentlemen named Anderson and Payne of the British Gem Testing Laboratory and named for the place it was first used at Chelsea University.
The filter was first developed for the separation of natural emerald from green beryl. At the time only emeralds colored by chromium were considered to be true emeralds. The emeralds colored by vanadium were considered to be green beryls. Since the chromium colored beryl would show the red color through the Chelsea filter and the vanadium would not, the filter was a diagnostic separations for natural emerald and natural green beryl.
How it works..
The Chelsea filter works by allowing only certain wavelengths of yellow-green and red colors to be transmitted through the filter. As a result, gemstones transmitting these colors will appear as either green or red, while other gemstones will simply appear dark. This allows, for instance, chromium based emeralds to appear red since the red color is transmitted and the deeper green is not allowed to pass. (only the yellowish-green). Likewise, in a green cobalt-coated topaz, the red of the cobalt is readily visible but the green color is not shown. Allowing a quick identification of a green stone as cobalt treated. The same applies to many other gemstones that can be viewed with a Chelsea filter. And while the Chelsea filter reaction can rarely be considered diagnostic, it is a very good indicator, and can quickly be used for diagnostic tests when used in conjunction with other gemological tools.