|Tourmaline has been called a junk mineral (look at the formula!) it is sort of made up of whatever is left over once other materials have formed. The tourmalines containing lithium are the most desirable for gem materials. It can be fund in virtually all the colors of the rainbow.
The natural crystals are strongly pleochroic (they have different colors or color strengths in different crystal directions.) This can be useful in designing the cut. Stones can be cut looking out of one of the crystal faces, and the C-axis color will then become a highlight color in the stone. See the example below (tourmaline 9) where the stones are dark blue-green with yellow-green highlights.
Sometimes the c-axis color is so strong that it cannot be use as the table of the stone or it will be so dark light will not be returned This condition is sometimes referred to as a "blocked" c-axis. These stones can still be salvaged by cutting at a right angle to the c-axis, but will still be dark on the ends or edges.
Red to pink tourmaline is called rubellite and is one of the most desirable stones. Rubellite does suffer from impurities and fractures. It is rarely found in large crystals that are clean.
There are several different variations in green tourmaline, some called "chrome tourmaline" contains chromium and is also highly prized for its beautiful green shade. Green tourmalines can be found throughout the range of greens from sea-green (blue-green), to middle green to yellow-green. All of the greens except the lightest ones are mid priced. The lightest greens (lowest color saturation are less expensive.)
Blue tourmaline is called "indicolite" and is very highly valued. It produces beautiful dark blue often with just a hint of green flash. The newest find in the tourmaline family is called "paraiba" tourmaline and was originally found in one mine in Brazil. It owes its unique "sky blue" color to traces of copper. It is a neon blue with very high brightness. It also commands astronomical prices. a new find has surfaced in Africa but it is till the most rare tourmaline.
There is also a wide variety of tourmalines in the yellow, yellow-orange to brown end of the spectrum. The more brown in he stone the less valuable. Yellow tourmalines and nice orange or red-orange stones are popular but tend to be more rare.
Violet colors, peach colors and many intermediate colors come and go, but have never really caught on compared to the major three colors, red, green and blue.
ONe point of interest in tourmaline is that it can often be found will multiple colors in the same crystal. AND the colors can run along the length of the crystal changing over the length from one to another, or they can change radially from the center out.
The radial ones are sometimes found with pink on the inside, a clear ring followed by green at the exterior of the stone these are often sliced across the stone to reveal the patten and are called "watermelon" tourmaline. (See tourmaline11 below).
When the colors change over the length of the stone the color change is often incorporated into the linear design. Many stones are cut with dual color or even tri-color. Examples are also shown below.