Tourmaline is a boron silicate mineral, which occurs as an accessory mineral in igneous and metamorphic rocks. There are a large number of varieties of tourmaline, each with a slightly different composition, resulting in a wide range of colours. Rossmanite has the formula (LiAl2)Al6Si6O18(BO3)3(OH)3OH and is is usually described as being pink in colour, however, this specimen from Cornwall has a grey green colour, and is composed of tightly compacted radiating sprays of prismatic crystals.