A dichromatic filter was developed in 1934 by B.W. Anderson & C.J. Payne at the Gem Testing Laboratory of London chamber of commerce and devised with the collaboration of Gemology students of the Chelsea College of Science & Technology. That filter was named as Chelsea Filter. It was developed to distinguish between emerald and its stimulants.

Chelsea Filter transmits both deep red wavelength (around 690 nm) and yellow – green wavelength (around 570 nm). This transmission matches emerald’s red transmission and yellow green absorption characteristics.

When emerald with chromium, as coloring element, is strongly illuminated and viewed through Chelsea Filter, Emerald will appear distinctly red or pinkish, depending on the color’s depth. Emerald’s red color in Chelsea filter is due to chromium present in the emerald and the amount of absorption of yellow-green and red portion of spectrum by the stone. This red appearance through Chelsea filter may diminish depending on the presence of Iron.

 Indian Gemological Institute – Gem Testing Laboratory, Delhi received one natural Emerald of 4.79 ct, SG 2.73, and RI 1.582 – 1.589, 0.007. (Figure no. 1) Under the magnification emerald showed usual inclusions like fingerprints, crystalline inclusions, phase etc. It was observed that emerald was hosting one more emerald crystal in the center. (Figure no. 2) Included crystal was possessing almost 50 percent of host emerald. It was appearing as if a bigger emerald crystal has grown enveloping one other emerald crystal of hexagonal cross section in between. Both of these crystals were having almost colorless area between each other, probably that was Beryl (as per the inclusions and appearance) (Figure no. 3).

Initially it was thought, if its mere a hexagonal color zoning. But close examination revealed that this center piece is a trapped crystal. As rough faces of inner crystal were slightly visible up to some depth, it was not a single crystal. Inner emerald crystal was observed little more transparent then host crystal and was having slight deep color.

Then the stone was checked under Chelsea filter by illuminating it with spectroscope’s strong light. (Figure no. 4) Inner Emerald crystal and host crystal appeared red under the Chelsea filter but the colorless portion between two appeared greenish with its blackish inclusions. (May be due to the color of Chelsea filter).

Photograph of the stone was not coming sharp so it was decided to take the photograph of stone’s appearance under Chelsea filter through microscope. To take the picture of whole stone it was decided to take the stone’s photograph with microscope, with vacuum tweezer which shall not hide stone’s edges.

But the reaction of host emerald in Chelsea filter changed when it was illuminated with the dark field illumination of the microscope. Center emerald crystal showed same red color in Chelsea filter but host emerald crystal showed green reaction instead of red, different from what it was showing earlier in spectroscope’s light (Figure no. 5). Even the colorless area was showing the same reaction as it was in spectroscope’s illumination. 

Such change has never been observed before by the author, so it was decided to search if some particular illumination is recommended ever to be used with Chelsea filter. But author could not find any such standard of illumination reported for the Chelsea filter. Only halogen strong light has been recommended to illuminate the stone while observing the stone under Chelsea filter.

To understand the difference in this reaction detail of both the light source used were checked.

Illuminating bulb used in the spectroscope was Dichroic reflector Lamp of Philips Company, this is a halogen lamp of 15 Volts and 150 Watts, Corrected Color Temperature (CCL) was 3400 Kelvin, and Color Rendering Index (CRI) was 100.

Illuminating bulb used in the microscope was Dichroic reflector Lamp of Osram Company, this is a halogen lamp of 12 Volts and 35 Watts, Corrected Color Temperature (CCL) was 3000 Kelvin, and Color Rendering Index (CRI) was 100.

Here, stone, Chelsea filter, camera were same and the difference in color reaction of the host crystal was observed with the change in Illumination. Its assumed that this difference in the reaction of host crystal was due to the change in illumination and particularity of their wavelengths. And if this difference in reaction of host emerald is due to illumination, do we require some standard illumination to be used to observe stone under Chelsea Filter?

[In Dichroic lamps heat emitted by the filament is transmitted out through the back of the reflector.]

The Emerald was also checked in Diamond View, if fluorescence difference between both parts (Host emerald and Inner emerald crystal) varies but inner emerald and host emerald showed distinct red fluorescence and white portion between the emeralds appeared inert (Figure no. 6).

While checking the emerald in spectroscope’s illumination, as the stone’s photograph was not coming sharp, it was thought to change the camera. It was interesting that when the stone was viewed through that second camera, stone showed the same reaction as it was showing in Microscope. Otherwise with bare eyes the stone was showing same reaction as shown in Figure no. 4. Host emerald was appearing green, inner emerald was appearing red and colorless area was appearing green (Figure no. 7). Here stone, illumination, and Chelsea Filter were same and the difference in the reaction of host crystal was observed with the change in camera. May be this change was due to the kind of lenses or filters used in the cameras or the selective absorption and transmission of wavelengths by the lenses.

Comments and suggestions are welcome.

(All photographs are taken by the author.)

Author:

Meenakshi Chauhan

Indian Gemological Institute - Gem Testing Laboratory (IGI-GTL),

Jhandewalan, Delhi

(Project of GJEPC)


LINK TO AUTHOR'S OTHER BLOG:

http://www.burstingbubblesinsapphire.blogspot.in/

http://phosphorescenceinsyntheticsapphire.blogspot.in/

http://www.gia.edu/gems-gemology/summer-2014-gemnews-prasiolite-brazil-law-twinning