Color Filters

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Color filters have many uses in gemology with the Chelsea Colour Filter™ (CF) as the most prominent of them.
Whilst the CF is the most used, other filters can serve for many other applications. Amongst those are diffused colored plates used in conjunction with a microscope to inspect sapphires, narrow bandwidth filters to determine dispersion and blue, red or yellow filters to examine fluorescence in gemstones.

Some people regard a CF as a primary tool, yet all modern writers disagree with that statement. It can however give clues to the identity of a gemstone when used as an additional (secondary) tool. One can never rely on observations with a color filter alone.

Most of these color filters show their real power when inspecting parcels of gemstones. When one examines, for instance, a parcel of blue sapphires with a CF and some stones would stand out bright red from the lot when viewed through a filter, one should be very suspicious as this is not a typical observation for sapphire.
As these filters are highly portable, they are used on locations where one could not take larger gemological equipment. Gem/mineral shows and flea markets are some of those locations.
At best the filters are there to arrouse suspicion or not, they tell you little about the identity of the gemstone.

Basic

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All color filters work on the same principle, they absorp certain wavelengths of light letting only a portion of the visible part of the electromagnetic spectrum pass through.

In the image on the right all colors of the visible range are absorbed (blocked) by a color filter except red. Thus letting only red light pass through. This lets us control light and it can be very useful when inspecting gemstones.

Proper use of colored filters

When used as standalone (so not with a refractometer or microscope), the filters should be held close to the eye as with a handlens. A strong allochromatic lightsource should be used to get as much light into the stone as possible. As with the spectroscope, the stone is best observed in reflected light with the stone table down on a non-reflective background.
Do not use fluorescent lightning as these lights contain little to none red parts of the electromagnetic spectrum.

Chelsea Colour Filter™

This filter was developed in the 1930's by Anderson and Payne in conjunction with the gemology class at Chelsea College in London, UK. The CF (as it is often abbreviated for "Chelsea Filter") lets only the red and yellow-green portions of white light pass through it.

It's origional use was as an Emerald filter, distinguishing between Emerald and its look-a-likes. Most Emeralds would appear red through the filter. With the comming of synthetic Emeralds, which also show red through the filter, its primary use as an Emerald filter is no longer valid. The red color seen through the filter is derived from the chromium content in the Emeralds and is widely available in its synthetic counterparts (although often the synthetic Emeralds may show a more vivid red through the filter). Also some natural Emeralds do not appear red through the filter.

However it has been proven to be usefull in other circumstances. With a few exceptions (as rare natural cobalt colored Spinel) hardly any gemstone is colored by cobalt. Most synthetic Spinels and blue glasses are.
These cobalt colored stones are standing out bright red in the CF. Some natural blue Sapphire may have traces of chromium, making them appear reddish aswell.
Natural colored Jadeite and dyed Jadeite may also be seperated by the same means.

Many gemstones have typical reactions to the CF and long lists are available in most standard gemological text books. The colors observed are usually red or green (or no reaction) in various shades of those colors. Usually the deeper the color of the stone the more vivid the reaction.

The many exceptions to the rule make it a tool of "early warning".

Emerald Filters

These filters, also known as "Hanneman-Hodgkinson Synthetic Emerald Filters", are additional tools to the Chelsea Colour Filters.
When a gemstone is identified as Emerald (natural or synthetic) by other means of identification, the stone is first examined through a CF. If the stone appears red, then the Emerald Filter is used. When the Emerald appears red through this filter aswell, it is synthetic. The motto is "red red, drop dead!".

Resources

  • Gemmology 3rd edition (2006) - Peter Read
  • Gem Identification Made Easy 3rd edition (2006) - A.C. Bonanno, Antoinette Matlins