A major factor in the color of gemstones is color centers. Color centers do not depend on trace amounts of transition elements to produce the color, but by damage to the gemstone’s structure. The creation of color centers is usually caused by radiation. The investigation of colors centers in tourmaline is difficult because tourmaline usually contains significant amounts of iron and manganese. Both elements produce very broad electron paramagnetic resonance or electron double resonance spectra which causes problem with testing.
Dr. Nassau of the Bell Labs published an article that indicated that colorless tourmaline could be changed into yellow tourmaline with gamma radiation in 1975. Sometime ago I found an article that followed up on Dr. Nassau observations. Its title is “Investigation of Radiation Induced Yellow Color Tourmaline by Magnetic Resonance. Its lead researcher is K. Krambrock and it was publish in Nuclear Instruments and Methods in Physics Research 2002. Any serious technical questions about the investigation of color centers in tourmaline should be directed toward this source.
After making as complete an investigation as I can, I have come to the conclusion that the color center O- is the cause of the color green in a significant amount of Paraiba/paraiba type/ cuprian tourmaline. I believe that the following facts support my contention.
1, Copper as a chromophore can not produce anything, but the cyan color in tourmaline.
2, Some greenish Paraiba/paraiba type/cuprian tourmaline can be heated to produce the cyan color in tourmaline by eliminating the yellow color originally found to the tourmaline.
3, Some Paraiba/paraiba type/ cuprian tourmaline have been found to contain extremely low levels of titanium, iron and manganese and still have a greenish color vector.
4. By the process of elimination, I can not find any other source of a yellow color in tourmaline that would meet the above criteria.
Comments solicited and appreciated.
Again, please consult the research paper for any technical details on what an O- trap is and how it is generated in tourmaline by radiation.