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 Post subject: Thoughts on tourmaline's dynamic response to light.
PostPosted: Fri Jun 07, 2019 2:13 pm 
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Joined: Thu Sep 25, 2008 10:33 am
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Location: Mars PA
To get started, I would like to state some points that may be relevant to my discussion on tourmaline's dynamic interaction with light.

1, People do not see colors identically. An example would the inability to distinguish red from green that we call color blindness. This maybe an extreme case, but even people with a "normal" color perception ability can endless debate on such changes as the color transitions from green to yellow in tourmaline and purple to red in tourmaline.

2, The need for color consistency has affected the evolution of the mind/eye combination in order to handle the variable nature of daylight. Artificial light certainly expanded the range of colored light sources, but the only light of interest here is a white light that principles varies in the amount of its yellow component.

3, The following is based on my personal observations with an attempt to be assisted by my spectrometer. Difficulties in positioning the light source caused by differences in tone value and the size of a gem, have limited the usefulness of the spectrometer. I hope to work with the vendor to come up with software that will make the determination of the color of the gemstone consistent.

Now I have written a lot on the effect on the color of some tourmaline when observed under different light sources. Some tourmaline show the effect of varying light sources than others. This leads to color shifters verses color changers that can get to be personal. The spectrometer can spot a color changer because the gem's absorption graph must have at least two absorption peaks. Whether the color change is enough to stimulate the eye to detect it is another personal question. So what do I have to add about tourmaline's response to changing light sources?

For sometime I have been tracking a small group of tourmalines in the pastel blue green range. Some of them are cuprian from Mozambique and some sea foam from Afghanistan. The focus on color in this range of tourmaline, as a determination of the tourmalines copper content, has certainly caused consternation in the trade and lost investments. High grade gems in both groups have a glow-like quality that I believe is superior in the copper bearing tourmaline, but they both have similar tone values. Both groups respond to increased yellow in their light sources with more vivid color and an apparently richer tone level. Their colors also shift to a relatively small degree, but that is not nearly as important as the enriching of their colors. (Other pastels show a similar effect when viewed in lower levels of defused natural light after sunset.)

So how do the two groups compare when the yellow in their illuminating lights reaches the level of incandescent light? The sea foam is beautiful in its vivid, well toned world, resplendent in its glow-like aura, while the cuprian is out of this world. It is everything I would want in a gemstone. Wow what a change in presence from just a change in the percentage of yellow in the light that baths its beauty. The Mozambique material is so sensitive to how yellowish the light being used to photograph it with, that I don't trust any any photographs to honestly show off its beauty without knowing how it was photograph. And I mean both the intensity of the light and its yellow content. Finally the claim of the use of natural light is not enough to judge the gemstone, because you can clearly see an exception difference in the cuprian tourmaline, from natures variable light source, the sun.

Bruce

The yellow content of the light is also the key factor in showing the reverse alexandrite color change in unheated cuprian tourmaline from Mozambique that I have come to call Laurellite.


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