Garnets are Red … Sometimes

January’s birthstone, garnet, is generally associated with the color red. But did you know garnets also come in orange, green, brown, and other colors?

Garnets are actually a group of gems with similar, but slightly differing chemical formulas.  The generic formula is A2B3(SiO4) where A is calcium, magnesium, iron or manganese (Ca, Mg, Fe, Mn) and B is aluminum, iron or chromium (Al, Fe,Cr). The different combinations of elements create the different species of garnet (listed below with their common colors). Color can be a very helpful diagnostic feature when identifying some (but not all) gems.  For instance, spessartine garnet must always have an orange color component.

min_garnet

Almandine garnet collected at Gore Mountain in New York (left), spessartine garnet crystal (locality unknown, right)

  • Pyrope: medium to dark reddish orange, red through slightly purplish red to pale pink, colorless
  • Almandine: brownish orange to brownish red to purplish red
  • Spessartine: orange, brownish orange, yellowish orange, reddish orange, and red-orange
  • Grossular: medium to dark orange, medium to dark green to yellowish green, colorless
  • Andradite: yellow to slightly greenish yellow or brown, green through yellow-green
  • Uvarovite: light to dark green through yellow-green to brownish green
  • Hydrogrossular: pink, green, brownish pink, white, gray

Because of the range of chemical formulas possible, garnets may also be mixes of more than one species.  In addition, some species have subspecies (called varieties).  Some examples are Demantoid (a variety of Andradite) and Hessonite (a variety of Grossular)

Some garnets also display “phenomena” such as stars, cat’s eye, and color-change due to features of that stone that affect how light is reflected and/or transmitted.

Other facts – Garnet: 

  • is the gemstone for those celebrating their 2nd wedding anniversary
  • is used as an abrasive
  • has been used in jewelry since ancient times
  • can be found in all three major rock types: igneous, sedimentary and metamorphic
  • has a hardness of 6.5-7.5 on the Moh’s scale

and comes in colors to please almost everyone!

GASSN102_a

Sources:

  • Gemological Institute of America, Gem Encyclopedia page on garnets: click here.
  • Gemological Institute of America, Gem Identification Lab Manual, 5/2012
  • Manual of Mineralogy, 20th Edition, Cornelis Klein and Cornelius S. Hurlbut, Jr., 1977.

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