A mineral group which all the gem references I can find say has the widest variety of colors of any gemstone. The color depends on what elements have made their way into the stone's structure. When not modified by a particular color, "tourmaline" usually refers to the green varieties; these and the red ones are the most valuable. A tourmaline can have more than one color in the same stone, such as the "watermelon tourmaline." The name "tourmaline" is actually derived from a Singhalese word meaning "mixed precious stones" -- unsurprising as Sri Lanka was the first source of tourmaline.

Cat's eye tourmalines are not too rare in any color. In addition to the color zoning, tourmaline is often dichroic. Some colors have specific names such as the blue-to-teal indicolite and the red rubellite, but other names given to types of tourmaline indicate a particular mineral composition rather than a color (schorl, elbaite, and dravite). Synthetic tourmalines are also available, and other stones are sometimes mislabeled as tourmaline.