Tourmaline is not the most dichroic gemstone of all gemstones or in fact intrinsically dichroic at all. I prefer to use dichroic rather than pleochroic because tourmaline can only express two colors in a crystal that are based on different axis of the crystal. I prefer to use pleochric for crystal that show different colors on all three axis like unheated Tanzanite.
All four mineral species, that are usually cut into gemstones, in the closely related group of minerals that mineralogist and gemologist call tourmaline, are intrinsically colorless and therefor not dichroic. These four minerals species are Elbaite, Liddicoatite, Dravite and Uvite, can not be distinguished by color or any other physical feature without appropriate testing equipment.
The level of dichroism that can be expressed in Elbaite, by far the most common mineral species used in gemstones that are referred to as tourmaline, can be from no absorption to complete absorption down the principle axis. (c axis). When the principle axis is in essences opaque in tourmaline the common way of saying that is, the tourmaline has closed ends. The most interesting feature of dichroism in tourmaline is not the spread of absorption between a colorless (achroite) and lets say a dark blue indicolite, but the tremendous difference between dichroic and non dichroic indicolite (other colors too) at all tone levels. This dichotomy, between non dichroic and dichroic tourmaline with the same color and tone level is important for both the cutting of a gemstone and identifying cut tourmaline. One aside should be made here, if the tourmaline is dichroic, the principle axis, (c axis) will be darker than the a/b axis. I use the designation of a/b axis as one axis because these axis must be the same color, because of the geometry of the tourmaline crystals.
So the mystery grows. How can an intrinsically non dichroic substance, show a complete range of dichroism, that is independent of the both the color and tone level of most tourmaline. The secret lies in the form of the chromophore that is incorporated into the tourmaline crystal. A chromophore is unusually a transitional element or combination of transitional elements that has the proper electronic configuration to absorb visible light and cause color in a substance. If the chromophore is in the form of an intervalence charge transfer, (IVCT), the internal structure of the tourmaline will alignment the axis of the IVCT with the c axis. An IVCT requires two atoms that interact to absorb visible light. The axis of the IVCT is a much more potent absorber of visible light than its other orientations.
Well there you have it. The wonderfully complex electronic nature of tourmaline is ready to orient some coloring agents that are strongly dichroic (IVCT) with its principle axis while other coloring agents are not imparted with any preferential absorption orientation, which results in beautifully open ended tourmaline.
I do have one unanswered question about the orientation of IVCT in tourmaline and that is whether all IVCT are aligned in all tourmaline under all conditions of growth. I do know that some tourmaline can have their axis of darkness not be aligned with the terminus of their crystals.