Tourmaline

Tourmaline is a group of similar minerals with similar chemical composition that forms in the hexagonal (trigonal) crysal system.   Gemologists don't usually differentiate between the different mineral species and typically refer to tourmaline by its color.  Tourmaline is found  in most colors, with blue, green, pink and red as the most well known.  Tourmaline can grown in large crystals and as chemical conditions change over a long period of growth the crystal can change colors so you can have two or more colors visible in a single crystal.