Tourmaline has a variety of colours. Iron-rich tourmalines are usually black to bluish-black to deep brown, while magnesium-rich varieties are brown to yellow, and lithium-rich tourmalines are almost any colour: blue, green, red, yellow, pink, etc. Rarely, it is colourless. Bi-coloured and multicoloured crystals are common, reflecting variations of fluid chemistry during crystallization. Crystals may be green at one end and pink at the other, or green on the outside and pink inside; this type is called watermelon.