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Colors of Tourmaline

(Text and Photos by Barry Bridgestock)

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     Tourmaline has become an increasingly popular gemstone because it comes in such a wide range of beautiful colors.  This range of colors is caused by its complex chemical make-up and its ability to attract and hold metal ions during crystal formation.  Vanadium 5+and chromium 3+ can result in a pure rich green.  If a crystal attracts iron and iron oxide it can result in a stunning sapphire blue.

    

The colors in a tourmaline can tell the story of its formation.  When the pink and white bi-colored stone pictured below was formed, the manganese necessary for the pink was not present at the beginning of the crystallization process.   

 The square cushion pictured below has even a stranger growth history.  The iron causing the attractive greenish-blue color when the crystal began to form was altered at the end of the growth process, giving the outer layers an unattractive khaki color.The attractive color was hidden inside.   

The medium green round pictured below underwent a similar process and the resulting piece of rough was so unattractive that I put off cutting it for an entire year!

 

Tourmalines usually form within pegmatites, which are coarse grained granites which have intruded into pre-existing rock formations under great pressure.  As the melt cools,  various types of crystals begin to form in pockets.  If the pockets are large enough and the cooling rate is slow enough, large crystals are able to form. Brazil is famous for its huge pegmatite veins with pockets large enough to allow the growth of huge crystals.  Fred Rynerson, who mined tourmaline in the early 1900's in southern California, described pockets measuring as much as 55 feet long and 40 feet wide in his classic book "GEMS & GOLD".    Tourmalines have also been found in contact-metamorphosed limestones, schists and gneisses but they are seldom of gem quality.

    Tourmaline is much more dynamic than other gemstones because of discoveries of new types and colors.  In the late 1980's, tourmalines containing copper were discovered in the Sao Jose Batalha area of Paraiba, Brazil and the colors of these stones were spectacular.  As the Paraiba deposits began to dwindle, beautiful blue and blue/green gems were found in Namibia, Nigeria and Mozambique.

 

Rather unusual color-change stones containing bismuth and/or copper were found in Mozambique in 2004 also.

In the last couple of years yellow and coppery-pink tourmalines have been found during the search for "Paraiba-type" gems in Mozambique.

 Recently I cut the red round brilliant pictured below from a red tourmaline rough that looked so much like a chrome-pyrope garnet that I immediately tested its specific gravity, pleochroism and absorption spectra prior to cutting.  Since it also shows a fairly strong reaction to the Chelsea filter,

I think more testing of this stone could be interesting.

   The seemingly endless color range of tourmaline will insure continued interest in this 'rainbow gem.'

      


                   

 GEMMOLOGICAL DATA

 

Make-up:  complex aluminum borosilicate with changeable composition

 Luster:  vitreous

 Hardness:  7.5   Suitable for all jewelry uses. 

Crystal structure:  trigonal, also considered hexagonal for reasons illustrated in the picture below.

        Fracture:  concoidal

 Cleavage:  none

 Density:  3.06

 R.I.:  1.616-1.652

 Birefringence:  .018

 Pleochroism:   moderate to very strong

 Dispersion:  .017

 


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