Tourmaline

Tourmaline forms primarily in granitic pegmatites - the last pockets of mineral-rich fluid that crystallize as a granite body cools. These pegmatite fluids are enriched in boron, lithium, and other volatile elements that don't easily fit into the crystal structures of common rock-forming minerals. As the fluid cools below about 600 degrees Celsius, tourmaline crystallizes, incorporating whatever trace elements are available.

This is why tourmaline comes in virtually every color. Iron produces black (schorl) and green. Manganese creates pink and red (rubellite). Copper - in the rarest and most valuable variety, Paraiba tourmaline - produces an electric neon blue-green unlike anything else in the gem world. Some crystals even show multiple colors, changing from green on the outside to pink in the center ('watermelon' tourmaline) as the chemistry of the fluid changed during growth.

Tourmaline is also piezoelectric and pyroelectric - it generates an electrical charge when squeezed or heated. This property was known to Dutch traders, who used heated tourmaline crystals to pull ash from their pipes, calling it 'aschentrekker' (ash puller).

Tourmaline's elongated, striated prismatic crystals with a distinctive rounded triangular cross-section are its most diagnostic feature. No other common mineral shows this cross-sectional shape. At 7-7.5 Mohs hardness, it's durable enough for all jewelry applications.

Color alone is unreliable for identification since tourmaline spans every hue, but the combination of crystal shape, hardness, and lack of cleavage is definitive. Many tourmaline crystals show pleochroism - they appear different colors when viewed from different angles. Looking down the length of a green tourmaline crystal, it often appears much darker than when viewed from the side.