TOURMALINE

Tourmaline owes its almost limitless range of colours to its complex chemistry. It is an Aluminium Boron Silicate with highly changeable composition, even within the growth cycle of a single crystal. The result is that a mineral specimen is rarely restricted to a single colour, with individual crystals containing two or more both along its length and from its outer borders to its centre, giving rise to varieties such as 'Watermelon Tourmaline' with its green 'skin' and a white band surrounding its red centre. 

Furthermore, Tourmaline may exhibit a change of colour when viewed from different angles due to its strong pleochroism. Green and red specimens are frequently heat-treated to enhance colour.

It can be found in many locales, with Brazil, Sri Lanka and Mozambique producing high quality specimens.
Varieties of Tourmaline include:
Rubellite:  Pink to red colour
Achroite:  Colourless (rare)
Dravite:  Yellow to dark brown
Verdelite:   Green
Indicolite:  Blue
Schorl:  Black (opaque)