Tourmaline's name is its own definition. Dutch traders in Ceylon (now Sri Lanka) took the Sinhalese word toramalli, loosely "mixed gems," and applied it to the multicoloured stones coming out of the island's gem gravels. Unlike most gemstones, tourmaline was named for its variety rather than for one colour or one place: the plurality was the point from the start.
The stone had already caused confusion by then. In the 1500s, Spanish explorers in Brazil found green tourmaline and mistook it for emerald, a misreading that held for centuries. It wasn't until the 1800s that mineralogists confirmed tourmaline as a distinct species altogether: a single mineral group capable of producing an unusually wide spread of colour from one crystal to the next, and sometimes within a single crystal at once. That internal variation, colour zoning along or across a stone, is what produces effects like watermelon tourmaline, a pink core ringed in green. It's a structural fact of how the crystal grows, not an embellishment.
A century later, in the late 1980s, prospectors in Brazil's Paraíba state found a copper-bearing variety unlike anything on the market: an electric, neon blue-green now considered among the rarest colours in any gemstone. Copper-bearing tourmaline has since turned up in Mozambique and Nigeria too, extending the group's range further still.
