All Change in the Color of Gemstones is NOT Color Change.

When I started cutting and collection tourmaline, its changes in color, under many different conditions, were far from my interest.  I wanted beautiful colors, and all of them, in a natural stone.  Years later as I was trying to get examples of all the beautiful colors coming out of an unknown location (to me) in Mozambique I discover examples of new variety of cuprian tourmaline that changes in blue in incandescent light to purple in natural light.  This is in the opposite direction to Alexandrite (The only color change gemstone that truly has an important place in the trade) which is unique in gemstones.  (and all other materials known to science).

In order to investigate this wonderful new gemstone, I call it Laurellite after my older daughter, Laura, because it changed my life,  I had samples submitted to the GIA.  When the laboratory work came back, after years of waiting, I not only had verification of the strong reverse Alexandrite color change that I had observed, but the gemstones turned out to be colored by copper, that first announcement of high quality tourmaline,with copper as a chromophore, from Mozambique.

Since the GIA investigation left many unanswered question, I sought out the lead researcher in a past GIA attempt to define color change in gemstones and survey the world of gemstones for color change.  His name is Yan Liu and the paper that was produced and published in a British Gemmological Journal (J. Gemm, 2006, 30,3/4, 201-206) with my gemstones and support is an important step in understanding the physical nature of the color change phenomenon.  My attempt to scientifically discuss changes in color in gemstones on this page is in part based on that published paper.  But the discussion has to go further because there seems to be a very poor grasp of, or agreement on, what the definition of color change in gemstones should be.

I will be building my definition of color  change in gemstones on work done by Liu,Y.,Shigley, J., Fritsch, E., and Hemphill, S., 1994.  The “alexandrite effect” in gemstones.  Color Research and Application, 19(3), 186-91 as interpreted by Liu,Y., as I talked about above.

The first step in the process of discussing color change is limiting the change in color to gemstones that possess one color under natural light and another color under incandescent light along one optical axis of the gemstone.  This broad statement needs to be more carefully defined in order to scientifically define color in a quantitative manor.

The poorly defined terms of natural light and incandescent will have to be replaced with luminants.  Illuminants are hypothetical light sources that are used to calculate hue angle in the CIE color world.  They can not exist in the real  world.

The CIE color world was a representation of every color man eye’s can see and has as its generator hypothetical colors.  It was developed at the turn of the 20th century and is well establish and used every day.  It is a very large color world, there are others, and it includes many colors that man can not see.

Hue angle is a way to help describe a color location in the CIE color world.  It is based on the use of polar coordinates and vectors.  The length of the vector is the saturation of the color, with a neutral completely desaturated color being the central origin of the polar coordinates.  (Brown and Gray or not spectral colors and have no meaning in defining a color in the CIE color world.)  Conventionally the axis that coincides with the x axis in orthogonal coordinates is both red and set at a zero angle.  Yellow being at a rotation of 90 degrees from red, with green being at 180 degrees from red and blue being at 270 degrees from red.  While purple, blue green. yellow green and orange fall in there appropriate slots as you rotate counter clockwise around the polar drawn grid.

With these definitions, we are ready to try a put a quantitative analysis in place for gemstones that should be defined as color change and those that do not qualify.

According to research done by Lui and associates, the eyes ability to retain color distinction between the color of a gemstone between illuminant A (approximately natural light) and D65 (an approximate incandescent light source) is a hue angle of 20 degrees.  This number is not hypothetical, but came from actually testing man’s eyesight.  Degree’s of color change depend on increasing hue angles with a 90 degree being the start of defining a strong color change.  Data for the hue angle determination should be obtained with a spectrometer that is properly calibrated, not by the eye.   Color that changes that occur because of the change in viewing angle of the gemstone should not included in the color change calculations.

The discovery of a reverse color change in a new variety of cuprian tourmaline that I have named Laurellite, has rendered the traditional and often cited theory, on why color changes in gemstone, inadequate.  It can not explain the reverse color change nature of Laurellite, which has a warmer color (purple) in a cooler light source (natural light) and a cooler color (blue to blue green) in a warmer color.  There is still the requirement for two peaks of absorption of visible light and differences in the spectral distribution of wavelengths in the two lights, but the adaptive power of the human eye must also be taken into consideration.  It is proposed that the eye input and the brain’s analysis of the input are adaptively combined to maintain color consistency.  The maintenance of color consistence under different light sources is extremely useful to man.  (Oranges will continue to appear orange even with the spectral changes that occur in natural light during the day.)  The two areas of the spectrum that are most effected by the non linear response of the eye-mind to change in the color balance of light are between the red and orange and blue and purple.

I have not gone into changes in color due to changes in the axis of observation and thickness of the gemstone, because they are not color change gemstones in the definition I presented here.  I do think that some changes in hue angle are much more interesting and rare than others with the with the very personal change between yellow to green and the natural instability of all purple gemstones.  Both of which depend on the nature of man’s eyesight.

 

 

 

Bruce

 

 

 

 

 

 

 

Leave a Reply