Caption

Black tourmaline. Coloured scanning electron micrograph (SEM) of a sample of black tourmaline, showing its elongate crystals. Tourmaline is a complex silicate of boron and aluminium, which occurs in many colours. The colour depends upon the other minerals it is composed of. Black tourmaline, also known as schorl, contains iron and is the most common form of tourmaline. Tourmaline is most often found in pegmatites, layers of course granite formed by the crystallisation of magma.