Use
Tourmaline, which is a borosilicate, is the main source of
boron found in soil. It is a mixture of silicates and borates
from where boron is extracted. It is available as a
complex cyclosilicate, Al6Y3Na[Si6O18](BO3)3(OHF)4,
with Y = Mg, Fe, Mn or Li. The rhombohedral system
has the shape of an elongated rod or needle type prism.
Tourmaline is used as a gemstone, and because of its
double refraction and piezo-electric properties, it is used
in polarizers and in some pressure gauges. The best
known variety of tourmaline is black tourmaline which
is rich in iron and is a common mineral in igneous and
metamorphic rocks.
Magnesium-rich tourmaline is called dravite.
Lithium-rich varieties are called elbaite. Ferruginous
varieties are called schorlite which is insoluble in water
and resistant to weathering and hence releases boron
slowly.