The Chelsea Color Filter

The Chelsea Color Filter was first developed by the Gem Testing Laboratory of the London Chamber of Commerce (now GAGTL/Gem-A) in 1934, and was first used at the Chelsea College of Science and Technology as an aid in the separation of emerald from its simulants. It was originally called an emerald filter.

The Chelsea filter consists of a combination of two gelatin filters that pass wavelengths in the deep red and in the yellow green. To obtain best results, stones should be viewed under a strong artificial light with the filter held close to the eye so as to cut out any outside glare of light. The gemological uses of this simple instrument are limited, but it can be useful in certain circumstances.

The chromium in emerald makes it appear reddish or pinkish when the stone is strongly illuminated and viewed through the filter. Emerald imitations that do not contain chromium appear green through filter. A few natural emeralds, especially those from South America, fail to show pink through the filter. Once synthetic emeralds appeared on the market, the Chelsea filter became less useful as an emerald filter, because the synthetic product, also containing chromium, appears red through the filter, though often of a deeper red than natural counterparts.

The Chelsea filter is also useful in separating cobalt-containing gem species from others, as the presence of cobalt results in a red reaction through the filter. Gemstones colored by cobalt are in the main synthetic or man-made, and appear red through the filter. This has proven useful in separating synthetic blue spinel and man-made blue glass from the stones they imitate, namely aquamarine, blue zircon and sapphire. Gemstones containing cobalt are rare on earth.

Aquamarine and its most common simulant, synthetic blue spinela are visually very similar, but owe their respective blue color to different causes. Aquamarine is colored by iron, which absorbs red wavelengths and transmits some green wavelengths, and appears green through the filter. Synthetic blue spinel is colored by cobalt which absorbs green wavelength, but transmits red wavelength, appears red through the filter.

The absorption spectrum of aquamarine, colord by iron, and synthetic blue spinel, colored by cobalt, can help us understand why their reactions to the Chelsea filter are so different.