Tourmaline
“The rainbow mineral”
Tourmaline is a boron silicate mineral group famous for occurring in virtually every color of the spectrum, sometimes multiple colors in a single crystal. Watermelon tourmaline, with its pink center and green rim, is one of the most striking examples of color zoning in the mineral world.
complex borosilicate (elbaite, schorl, dravite, and other species)
Minas Gerais (Brazil), Pala (California, USA), Madagascar, Afghanistan, Elba (Italy), Nigeria
7-7.5
Tourmaline is pyroelectric and piezoelectric — when heated or squeezed, it generates an electrical charge, which is why it was historically used in pressure gauges for submarine depth measurement.
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