Tourmaline gemstones can be found in all colors of the rainbow. Owing to its wide range of color availability, tourmaline is considered to be one of today's most versatile gemstones.Tourmaline crystals form in the trigonal crystal system and can be distinguished by their distinct three-sided triangular prisms.Tourmaline can occur in a wide range of colors from colorless to black. Colorless tourmaline is considered to be the rarest, but it is also the least valuable tourmaline. Black tourmaline is actually the most common color occurrence. Most tourmaline gemstones display two or more colors in a single stone (or two tones of the same color). Tourmaline crystals that exhibit green on one end and pink to red on the other, with a band of white in the middle are marketed as 'watermelon tourmaline'.Tourmaline is typically transparent to translucent. The level of inclusions can vary depending on the type of tourmaline, with some colors being more heavily included than others. Green tourmaline is often eye-clean, while blue, red and pink tourmaline, including rubellite, Paraiba and watermelon tourmaline, are almost always found with significant inclusions. Rubellite, Paraiba and watermelon tourmaline are considered to be Type III clarity gems.