I love tourmaline. It keeps coming up with exceptions to the rules/definitions.
Definition. A tricolor tourmaline has three different colors as seen from a point of view perpendicular to the principle (c) axis. It doesn’t depend on tourmaline’s dichroic nature at all, which can produce different colors depending on the angle of the point of view, to the principle (c) axis, that is being examined.
The type of tourmaline that I just saw on the inter net is an example of a beautiful tourmaline that shows three colors and is neither a tricolor nor depends on dichroism for its advertised “tricolor”. The specimens I have came from Nigeria and are a moderately pale pink with areas of yellow green of similar tone value, to the pink. The impurity that renders the area of the crystals green appears to have naturally diffused into the pink body of the tourmaline and dominates its locations of color in the crystal. Now that we have two colors that can be seen from all angles to the principle (c) axis because the material is not noticeably dichroic, how do we get the third color which is a yellow. Well years ago it was shown that when you mix green and red together you get a brown to yellow color depending on their relative tonal values. The yellow in this tourmaline can be seen as you rotate the gemstone because of the mixing action of the facets and is completely independent of the axis of view. You really don’t need much green tone to make it happen with the Nigerian material I have and the yellow color is not muddy as some mixes of color in tourmaline can tend to be.
The specimen on the inter net was one of the few gemstones on display that had any auction interest for it. The experienced dealer had not taken the time to accurately describe the gemstone along with completely inadequate pictures, so it is anybodies guess whether the purchaser will be happy with the gemstone.