I am sure that I have probably said too much about Laurellite on this site for some people, but questions have arisen and I wish to list the salient points about Laurellite that are important to appreciate it.
1, Laurellite reverse color change phenomenon is not only unique in gemstones, both natural and man made, but to all substances known to man. This is according to Yan Liu who did extensive work on the color change phenomenon for the GIA and work with my material for the last paper published on Laurellite. (A colorimetric study of tourmaline from Mozambique which show a reverse alexandrite effect. Published in the Journal of the British Gemmological Society 2006, 30, 3/4, 201-206).
2, Laurellite is a cuprian Elbaite that contains both copper in the plus two oxidation state and manganese in the plus three oxidation state. These are the only essential chromophores in Laurellite. This combination of chromophores produces a blue/purple under mixed light and a blue to blue green under incandescent light which changes to violet in natural light.
3, The unique change from a warm color, violet, under a cool light source, (natural light) to a cool color, blue green, under a warm light source(incandescent) is why it is referred to as a reverse Alexandrite color change tourmaline. Why some cuprian tourmaline display this phenomenon while other tourmaline with similar chemical properties do not, is unknown at this time. (When I speak of a warm or cool light, it has to do with the relative amount of red/blue in the light, not the temperature of the light source. Scientific work done in color analysis uses “luminants” which are hypothetical light sources that simplify the analysis, but can not exist in the real world.)
4, Laurellite is not only rare, like all tourmaline that has copper as a significant chromophore, but it is getting rarer. It is a very common practice to heat blue purple cuprian tourmaline, like Laurellite, from all sources of cuprian tourmaline, to reduce manganese in the plus three oxidation state to the plus two oxidation state. This eliminates the red factor in the blue purple tourmaline and produces a cyan color that is solely dependent on the concentration of the copper chromophore. (This assumes that there is no other chromophore, such as iron, involved with the gemstone’s color). The driving force for heating the material is the possibility of increasing its value many fold.
5, Laurellite is a “smoking gun” when it comes to theories about why materials change color under different “white” lights, like natural and incandescent light. Its reverse color change phenomenon can NOT be explained by the nearly universally accepted theory, that uses ruby, alexandrite and emerald as the ultimate example of why some color gemstones display the phenomenon of color change. (All three are colored by chrome in the same oxidation state, but under different charge/structural environments in each gemstone) Laurellite does have the minimum of two absorption peaks that a color change material needs to have, to be strongly effected by the changing distribution of waves lengths in different “white” lights. But in addition to this factor, the non linear response of the human visual system, to the varying intensities of the different wavelengths, must be considered. Without the adaptation of the eye as it evolved to maintain color consistence in our natural world the reverse color change of Laurellite can NOT be explained.