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 stand alone (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") allows only the red and yellow-green portions of white light pass through it.

It's original use was as an emerald filter, and distinguished between emerald and it's look-a-likes. At the time, only beryl colored by chromium was considered an emerald, beryl colored by vanadium was not. Most emeralds would appear red through the filter. With the advent 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 emerald 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 due to Iron content.

However it has been proven to be useful in other circumstances. With a few exceptions (such as rare natural cobalt colored Spinel) there are almost no gemstones that contain cobalt. Most synthetic Spinels and blue glasses do.
These cobalt colored stones appear bright red when seen through the CF. Some natural blue Sapphire may have traces of chromium, making them appear reddish as well.
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!".

The Emerald Filter is shipped with a "support filter" for some rare occassions.

Tanzanite Filters

This filter is created by Dr. Hanneman to discriminate between Tanzanite and its simulants as synthetic Forsterite. It is made of two filters, a color filter and a dichroscope (London dichroscope type with two polarising sheets).
The stone is inspected first with the color filter part of the device and then viewed through the dichroscope part. Tanzanite will give you a unique combination that is not observed in its simulants, pinkish-orange through the color filter and purplish-pink/blue through the dichroscope.

Other Color filters

There are some other filters that work on the same principle like the Ruby, Aquamarine and Jade filters. These may be useful for your particular needs, but like most others they are not primary tools.

Richard W. Hughes introduced blue and green filters in conjunction with an immersion cell to inspect color zoning in Corundum.
When one would inpect a yellow sapphire for color zoning under the microscope, one would immerse it in an immersion cell with methylene iodide. As the yellow of the light, the yellow of the immersion fluid and the yellow of the gem are not the best conditions to spot yellow zoning, a blue filter is placed just above the illumination to counteract the yellow in the lightsource. This will enable you to see colorzoning much easier.
For yellow and orange stones a blue filter is used, for red stones a green filter is used.

Crossed filters technique

Copper Sulphate solution in a flask and a red filter

The "crossed filters" technique should not be confused with "crossed polars" or "crossed polaroids" as they have to do with polarization, not luminescence.

A flask is filled with hydrous copper sulphate and white light is shone through the solution. The light exiting the solution will be blue. While illuminating the gemstone with this blue light, a red filter is placed between the eye of the observer and the stone. If the stone appears red when viewed through the red filter, this is clear proof that the stone is fluorescent in daylight.

The activator in the gem which causes this is the presence of Chromium (Cr) in the crystal lattice and this effect is predominantly seen in Ruby, Alexandrite, Emerald, red Spinel and pink Topaz. It should be noted that Iron (Fe) can greatly diminish or completely eliminate this fluorescence effect. As synthetic materials usually carry more Cr and little to none Fe, this glowing of red light is more intense than in their natural counterparts (in general).

The hassle of carrying hydrous copper sulphate is luckily eliminated by the invention of blue LED pocket (or keychain) torches that may be purchased for just a few USD at your local hardware shop. One could use a sheet of red selenium glass as the red filter, or even your Chelsea Color Filter. Other sheets like plastics could also serve as crossed filters.

With a sheet of blue material in front of one's light source, one can mimic the copper sulphate solution and/or the LED torch.

Using the same light source in conjunction with a spectroscope, one can then easily distinguish between Ruby and red Spinel.

Related Topics

Resources

  • Gemmology 3rd edition (2006) - Peter Read
  • Gem Identification Made Easy 3rd edition (2006) - A.C. Bonanno, Antoinette Matlins
  • Crossed Filters revisited - D.B.Hoover and B. Williams, The Journal of Gemmology, July/October 2005

External Links