Tourmaline
Tourmaline, a mineral used as a gemstone and for industrial purposes. Tourmaline is an aluminum borosilicate, consisting of aluminum, silicon, oxygen, boron, hydrogen, and varying amounts of other elements. It usually forms crystals that are shaped like a triangular prism with slightly curved sides.
Tourmaline has a glasslike luster, and is transparent or translucent. It may be colorless, white, pink, red, yellow, brown, green, blue, or black. Single crystals may show combinations of various colors—for example, pink, white, and green. Black tourmaline, the most common variety, is called schorl. Green tourmaline is known as Brazilian emerald; pink or red tourmaline, as rubellite. Green and pink varieties are often used in jewelry.
When a beam of light is passed through a transparent crystal of tourmaline, polarized light emerges—that is, the beam of light that emerges from the crystal consists of light waves that are parallel to one another. When a tourmaline crystal is placed under pressure, the ends of the crystal acquire electric charges proportional to the pressure. Because of its properties, tourmaline is used in some optical instruments designed to produce polarized light and in some types of pressure gauges.
Tourmaline is usually found in granite and other igneous rocks, and in metamorphic rocks, such as schists and crystalline limestones. Major tourmaline deposits are located in Afghanistan, Brazil, and Madagascar.
Chemical formula: XY3Al6(BO3)3Si6O18(OH)4, where X represents sodium or calcium and Y represents aluminum, iron, lithium, or magnesium. Specific gravity: 3.0 to 3.25. Hardness: 7 to 7.5.