Animals Plants People Tourmalines are abundant, complex boron and aluminum silicate minerals of variable composition. Tourmaline comes in a wide range of colors, depending on its mineral composition. Black is the most common. Colors can vary from clear, to pink, rosy-red, yellow, honey, green, blue, violet, brown and black .
Some crystals are pink at one end and green at the other.
Concentric color zoning often occurs as well. The colored varieties, when transparent and flawless, are cut as gems. The best-developed tourmaline crystals are found in pegmatites and metamorphosed limestones in contact with granitic magmas, as well as certain sedimentary deposits. In addition to its use as a gem, tourmaline is employed in pressure devices because of its piezoelectric properties. Pieces cut perpendicular to the long axis can polarize light, and a pair of these plates form a very simple polarizing apparatus known as tourmaline tongs .