Chelsea filter

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A Chelsea filter is a gemmological dichromatic filter used for identifying colored stones.

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[edit] History

The chelsea filter was originally devised by Anderson and Payne in 1934 at the Gem testing Laboratory of the London Chamber of Commerce & Industry. The filter was devised with the collaboration of gemmology 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 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.

Synthetic emeralds were commercially introduced around 1940. These produced the same pink-red response as some emeralds through the Chelsea filter. However, although this filter was unable to predictable discriminate between natural and synthetic emerald, it was subsequently found capable of distinguishing aquamarine, blue topaz and their blue synthetic spinel simulants, for unlike natural gemstones blue cobalt-containing synthetic spinels emitted a red fluorescence under white light.

[edit] Use

Hold the filter an inch or two from the eye. Light the stone with a strong light bulb or torch. The stone may appear to change colour. The filter must be held near to the eye but there is no need to hold the filter close to the stone, even items in showcases can be examined providing they are lit by strong lights.

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