Schorl Tourmaline is a mineral with a hardness of 8 out of 10 on the Mohs scale of mineral hardness [?]. These Trigonally structured gems are made of complex borosilicate, their full chemical compound being NaFe3Al6(BO3) 3Si6O18(OH)4.
Schorl is the black, opaque, sodium iron rich variety of Tourmaline, a complex borosilicate with a trigonal crystal system.
Tourmalines usually occur as long, three-sided prisms, which often have well-terminated ends. Sometimes they are found as parallel or radiating groups of long, thin prisms with striated ridges lining its surface. Its varieties span the widest color ranges in the mineral world. There are tourmalines in all possible colors and tints and in every mixture of shades one can think of. Unlike the silicate rings in beryl, the unit rings in tourmaline are all oriented with the tetrahedral points in one direction, which gives it the polarity seen in the crystal form and in its electrical properties.
Schorl is tourmaline's most widespread variety, found in granite pegmatites that have solidified slowly, together with a wide range of minerals, including beryl, zircon, quartz, and feldspar. It may be found as black radially clustered sprays called "tourmaline suns."
The most famous deposits are in Sri Lanka, the Urals (Russia), Afghanistan, Burma, the USA (California, Maine, Connecticut), Brazil, Tanzania, Zimbabwe, and Namibia. Black tourmaline masses up to 7 x 10 cm, called colloform schorls, which formed in silica-rich rock without any melting taking place, such as in ore deposits in Azerbaijan.
A well-known specimen is the Huge Tourmaline Crystal, a black tourmaline or schorl weighing 8 kilograms, from Overlook, New York, USA, found in 1942.
Another name for schorl is africita, Spanish for black tourmaline.
Schorl derives its name from the old mining term for black tourmaline found in a tin mine in a German village near the western Ore Mountains, called Zschorlau.
When Dutch travelers to the Indies brought back the first tourmaline stones to Europe in 1703, they brought the original name "turamali," from the Singhalese (Sri Lanka). The Dutch knew the extraordinary property of tourmaline, its tendency to become electrically charged when rubbed or heated. Later Benjamin Franklin, researcher, writer and statesman, did extensive research on the properties of tourmaline in the process of forming his theories and inventions using electricity.
Schorl is the most commonly-occurring variety of tourmaline, and has no value as a gem, except sometimes for mourning jewelry. But specimens of schorl crystals in its natural rock mass are popular with collectors
It is sometimes used as an imitation of jet, and is called "jet stone".
A distinctive physical property of tourmaline is its pyroelectricity. Through warming or rubbing the crystal an electric charge is produced, with one end positive and the other negative. When dusted with a mixture of powdered yellow sulfur and red lead powder, the sulfur is attracted to the positive end and the red lead to the other. Like quartz, tourmaline also exhibits piezoelectric properties so it has an important application as a frequency stabilizer.