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A strongly dichroic, transparent crystal that is occasionally used as a Gemstone. Cordierite is a silicate of Magnesium, Aluminum, and sometimes Iron. It exhibits a dark, purplish-blue color from one direction and a pale yellowish gray color when turned 90 degrees. Cordierite is thought to be the sunstone whose strong polarizing effects were used by the Vikings to navigate their boats. Gem quality stones are mined in northern Europe, Sri Lanka, Myanmar, Madagascar, India, Canada, and the U.S. (Alaska, Wyoming, Connecticut). Cordierite is made synthetically at high temperatures from Clay, Talc, and Alumina.


Synonyms and Related Terms

cordierita (Esp.); cordierite (Port.); Cordierit (Deut.); cordieriet (Ned.)



Physical and Chemical Properties

  • Orthorhombic crystals but usually massive with embedded grains.
  • Good cleavage in one direction.
  • Fracture = conchoidal to uneven.
  • Luster = vitreous.
  • Streak = colorless.
  • Strongly dichroic from yellowish brown to blue at 90 degrees
Composition (Mg, Fe)2Al4Si5O18
Mohs Hardness 7 - 7.5
Density 2.5-2.8 g/ml
Refractive Index 1.53; 1.55; 1.55

Resources and Citations

  • G.S.Brady, Materials Handbook, McGraw-Hill Book Co., New York, 1971 Comment: p.794
  • Jack Odgen, Jewellery of the Ancient World, Rizzoli International Publications Inc., New York City, 1982
  • Robert Fournier, Illustrated Dictionary of Practical Pottery, Chilton Book Company, Radnor, PA, 1992
  • R.F.Symmes, T.T.Harding, Paul Taylor, Rocks, Fossils and Gems, DK Publishing, Inc., New York City, 1997
  • Encyclopedia Britannica, Comment: "cordierite" [Accessed December 11, 2001]. (color photo)(tech info)
  • C.W.Chesterman, K.E.Lowe, Audubon Society Field Guide to North American Rocks and Minerals, Alfred A. Knopf, New York, 1979
  • The American Heritage Dictionary or Encarta, via Microsoft Bookshelf 98, Microsoft Corp., 1998

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