#grwoth

Bicoloured tourmaline

The grab bag tourmaline family (as mineralogists sometimes name tourmaline) grows in pegmatites, those last stewing remnants of boron rich granites that slowly crystallise into a wide diversity of crystals, depending on the chemistry and evolution of the parent granite, and the magma's interaction with the surrounding country rocks into which it intruded. It forms in a wide variety of colours and slightly varying chemical compositions, being subdivided into several sub varieties with names such as Elbaite (named after the eponymous island that hosted Napoleon's first exile). Tourmaline often forms in multiple colours within the same crystal, sometimes in concentric layers, others in successive ones down the long axis of the crystal, as in this wonderful 31cm specimen from Brazil. This occurs due to changing chemistry and oxidation conditions in the mineral's mother ichor during growth. In this case, pinks, blues and greens are all due to iron. Tourmaline is also able to generate an electrical charge when heated, jewellers all know that they gather dust in sunny windows through static electricity at a rate an order of magnitude greater than other gemstones.

Loz