Cochise College Student Papers in Geology

Geology Home Page physical geology historical geology planetary gems

Roger Weller, geology instructor

Spinel
Paul Foss

Physical Geology

Fall 2006

Spinel

Photo by R.Weller/Cochise College

Spinel is a metamorphic material that’s name is thought to have come from the latin word “spina” or “thorn”. Red spinels are sometimes called “balas rubies” due to their similarity to rubies in appearance. It is used for jewelry as well as some commercial uses. Its chemical composition is MgAl2O4 or Magnesium Aluminum Oxide. It has a hardness of 8.0 making it an ideal gemstone as it will not break easily.

Gemstone

Spinel is used as a gemstone and comes in a variety of colors: purple and green, as well as a variety of colors ranging from pink to red to black.

Photo by R.Weller/Cochise College

High quality red spinels have often been mistaken for rubies and have been used as crown jewels. One of the most well known is the Black Prince’s Ruby one of the British Crown jewels. The Black Prince’s Ruby is a 140 carat spinel which is mounted on the Imperial State Crown of Great Britain.

Formation

Spinel is usually found in fractures in baked limestone. Often times spinel is found alongside corundum as the environment it is made is in is the same that produces corundum.

Photo by R.Weller/Cochise College

Spinel atoms form octahedral shaped crystal structures and are packed tightly together. They have imperfect octahedral cleavage and a conchoidal fracture.

Value

Gemstone quality spinels are usually have a reasonable price and are not as expensive as the more popular diamonds and rubies. Red and pink spinels are often times more expensive than the blues, greens, and other colors.

Works Citied

http://www.gemstone.org/gem-by-gem/english/spinel.html

http://www.geo.utexas.edu/courses/347k/redesign/gem_notes/Spinel/spinel_triple.htm

http://www.theimage.com/gemstone/spinel/spinel.html

http://www.gemhut.com/spinel.htm

http://skywalker.cochise.edu/wellerr/mingem/gemtp/spinel/purple.htm

http://skywalker.cochise.edu/wellerr/mingem/gemtp/spinel/spinel-bgB.htm

http://skywalker.cochise.edu/wellerr/mineral/spinel/spinel5.htm