Copyright © Barbra Voltaire, FGG, G.G. 1999
It is important to recognize what a Chelsea Filter IS and more importantly, what it ISN'T.
Chelsea Filters are simply dichromatic filters developed in 1934, not originally intended to be used for gemological purposes.
The filter is only able to transmit light in 2 regions of the spectrum: deep red (690nm) and near yellow-green (570nm).
Therefore, an object can only appear red or green or a brownish tone, resulting from a combination of red and green, when viewed through the filter.
It was discovered (in 1934 ) that one could gemologically separate some natural emeralds from imitations and other green gems by observing them through the filter.
That is because many Colombian and Siberian emeralds absorbed most of the yellow-green portion of the spectrum, but NOT the red portion of the spectrum. Therefore, when viewed through an apparatus only allowing transmission of red or yellow-green, they appear red.
This characteristic is NOT true with all emeralds. Typically African emeralds and Indian emeralds absorb portions of the spectrum around 690nm, so they will NOT appear red when viewed through a Chelsea.
When one viewed green glass, green sapphire, many green tourmalines, etc through the filter, they did not appear red as these stones were absorbing red color around 690nm.
BUT there are varieties of green flourite, green zircon and demantoid garnets which also absorb the yellow-green portion of the spectrum while transmitting red.
Subjective determinations were assigned to the Chelsea. "Colombian Emeralds appear more red".
OK....as a gemologist, I could easily and POSITIVELY separate all the gems listed above using a refractometer, polariscope and some magnification.
With the development of synthetic emeralds in the mid to late 20th century, it was observed that ALL synthetics appeared red when viewed through a Chelsea, as they were colored solely by chromium, thus absorbing in the yellow-green and transmitting the red.
It had been subjectively asserted, again, that these synthetics appeared "more red."
Well, more filters were developed to separate natural and synthetic emeralds.
Would I bet my professional reputation on the basis of subjective observations made with these filter reactions?
In addition, it is reported that some blue materials colored by cobalt appear red when viewed through a Chelsea. That is because cobalt can cause the absorption of the green portion of the spectrum, while allowing transmission of the red. This was considered useful in helping to separate synthetic cobalt colored blue spinel, blue cobalt glass, or doublets which may be confused with sapphire in the mid 20th century. But bear in mind, a sapphire containing chromium as a chromophore can also appear red, as can some natural blue spinel.
Again, standard gemological testing equipment would be far more reliable.
Chelsea Filters were also used to help separate aquamarine and natural zircon from synthetic flame-fusion spinel (used extensively in "birthstone" jewelry}, as both of the former absorb the red portion of the spectrum and the synthetic spinel did not.
Again, is there anyone here who would need a filter to make that separation?
Over the years folks have tried to ascribe the Chelsea with vast diagnostic attributes it simply does not have.
I think it is time to recognize that although a Chelsea may have been marginally (and I emphasize MARGINALLY) useful to the jewelry industry in the mid-20th century, it really has little professional benefit at this point.
Filters are confirmative, not determinative.
What does this mean?
If one has a parcel of stones, let's say, represented as aquamarine, and some of the stones appear RED when viewed with a chelsea, we have confirmed, they are not aquamarine.
BUT, we have neither determined what they are nor the identity of the rest of the stones in the parcel. They may be aqua.....they may not.
Chelsea Filter Reactions
compiled by Cheryl Castner
The Chelsea Filter is a tool that helps confirm what something is NOT. It can not identify what something is. Use it with reservation, as all reactions should be confirmed with other more reliable gem identification instruments.
|Emerald (some Emeralds from South Africa and India may not show a Red hue, but remain Greenish)||Pink to Red|
|Peridot||Green (Aqua Blue)|
|Soude Emerald (the old type Soude emerald may show red)||Green|
|Synthetic corundum (alexandrite effect)||Red|
|Synthetic emerald||Strong Red|
|Synthetic Spinel (some old types may show green)||Red|
|Tourmaline (Certain anomalous green tourmalines have been found to show Red – which would indicate chrome tourmaline)||Green|
|Garnets, dark red, no fluorescence||Red|
|Garnet topped doublet , no fluorescence||Dark Red|
|Glass, paste, no fluorescence||Reddish|
|Ruby, natural and synthetic, strong fluorescence The natural and synthetic sapphire are indistinguishable under the color filter)||Red|
|Spinel, synthetic, fluorescent ( pink synthetic spinel does not show a red color through the filter)||Red|
|Garnet topped doublet||Greenish-blue|
|Glass, paste, dark blue||Red|
|Glass, paste, light blue||Greenish|
|Lapis lazuli||Weak brownish red|
|Sapphire, blackish (The Blue sapphire which shows a Purple color under artificial light, usually shows Red under the filter)||Green|
|Spinel, colored by cobalt||Red|
|Swiss lapis, greenish-blue||Greenish blue|
|Synthetic sapphire (The natural and synthetic sapphire are indistinguishable under the color filter)||Dark Greenish Blue|
|Synthetic Spinel,dark blue||Red|
|Synthetic Spinel, light blue||Orange|
|Synthetic Spinel, zircon color||Orange to Red|
|Synthetic Spinel, lapis lazuli color||Bright Red|
|Violet Sapphire||Bright Red|