tourmaline

tourmaline (tŏrˈməlĭn, –lēn) [key], complex borosilicate mineral with varying amounts of aluminum, iron, magnesium, sodium, lithium, potassium, and sometimes other elements, used as a gem. It occurs in prismatic crystals, commonly three-sided, six-sided, or nine-sided, and striated vertically. Different crystal forms are usually present at opposite ends of the vertical axis. The luster is vitreous. Colors are red and pink (rubellite), blue (indicolite, or Brazilian sapphire), green (Brazilian emerald), yellow, violet-red, and black (schorl). Colorless varieties are called achroite. Two or more colors may occur in the same stone, the colors being arranged in zones or bands with sharp boundaries between them. Some Brazilian stones have a red core with a green exterior, separated by a colorless band; some stones from California are green within and red outside. The variations in color are, of course, dependent on the variations in chemical composition. Tourmalines are found in pegmatite veins in granites, gneisses, schists, and crystalline limestone. Sources of the gem include Elba, Madagascar, Myanmar, Sri Lanka, the Urals, Siberia, Brazil, and Maine, Connecticut, and California in the United States.

The Columbia Electronic Encyclopedia, 6th ed. Copyright © 2012, Columbia University Press. All rights reserved.

More on tourmaline from Fact Monster:

  • Birthstones - Birthstones Month Stone January Garnet February Amethyst March Aquamarine or Bloodstone April ...
  • Benjamin Wilson - Wilson, Benjamin Wilson, Benjamin, 1721–88, English portrait painter and electrician who ...
  • Franz Ulrich Theodosius Aepinus - Aepinus, Franz Ulrich Theodosius Aepinus, Franz Ulrich Theodosius , 1724–1802, German ...
  • cat's-eye - cat's-eye cat's-eye, gemstone that displays a thin band of reflected light on its surface ...
  • polarization of light: Polarization Techniques - Polarization Techniques Unpolarized light can be converted into a single polarized beam by means of ...

See more Encyclopedia articles on: Mineralogy and Crystallography