Tanzanite

by
Jessica Berg

Image taken from:
http://www.jewelrycentral.com/Target_Tanzanite.html

Table of Contents


Introduction
Physical Properties
Crystallography
Chemical Properties
Localities
History
Other Zoisite Minerals
Gemstone
References


Image taken from:
http://www.tradeshop.com/gems/zoisite.html

Introduction

Tanzanite is a zoisite mineral that is fairly new to the jewelry market.  It was discovered in the 1960's in Africa(1).  Tanzanite is now a popular blue/purple gemstone in America and has even been placed in the same category as ruby, sapphire, emerald and opal in some markets.  Its tremendous popularity, along with hand mining limitations, flooding, and government challenges, has created a shortage of this beautiful mineral.

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Physical Properties

Tanzanite has a hardness of 6.5. It is blue to purple in color, with a white streak.  The luster of tanzanite is vitreous and it is transparent to translucent.  It has a specific gravity of 3.2 to 3.4.  Tanzanite has good one directional cleavage and uneven to conchoidal fracture.  Tanzanite occurs in several habits, including prismatic, striated, and columnar.

Tanzanite is trichroic or when viewed in various orientations, different colors result(2).  In one direction it is blue, in another, purple, and in a third direction it appears bronze.  These colors are more intense in larger pieces of tanzanite (over 10 carats). The smaller ones are usually more pale.(1)  This property makes tanzanite easy to identify because it is a rare property.

Tanzanite looks different in different lighting situations.  Sunlight usually makes it look more purple.  Light bulbs and candlelight also bring out the purple and can make the grayish ones look brown.  Fluorescent lights tend to bring out the blue more, as do overcast skies.(2)

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Crystallography

Tanzanite is in the orthorhombic crystal system. The crystal class is 2/m2/m2/m, while its space group is Pnmc.  It has common forms of {100}, {111}, and {001}.

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Chemical Properties

Tanzanite is a hydrated calcium aluminum silicate mineral with a chemical formula of Ca2Al3Si3O12(OH).  It is 18.76% calcium, 12.63% aluminum, 19.71% silicon, .24% hydrogen, and 48.67% oxygen.  Tanzanite has a molecular weight of 427.38 gm.

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Localities

Geologically, tanzanite is found in metamorphic rocks, pegmatite rocks, and hydrothermal veins. Geographically, tanzanite is found in only one place in the world, Merelani Hills of Tanzania around Mount Kilimanjaro.  Specifically, in Merereani is in the Arusha province, which is located in the Simanjiro District of Northern Tanzania.  It is south of the Kikuletwa River and about ten miles from the Kilimanjaro International Airport.  The mines are now at a depth of 140 meters and cover an area of over 350 meters underground.  Crystal quartz, pyrite, and various geodes have also been found in the mines.(3)

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History

According to legend, tanzanite was first discovered by some Masai herders when they stumbled on some blue crystals lying with some other rocks.  They reported that the crystals had become blue when lightning caused a fire that swept through the area and heated the zoisite, changing the brown color of the crystals to the wonderful blue/purple.(1)  The first person to identify this blue zoisite was George Kruchik in the early 1960's, who was given some samples that were supposedly blue sapphire.  However, Manuel D'Souza is the person credited with discovering the deposits where it is now mined.  He found them while prospecting for rubies.  Shortly thereafter, the Tiffany Company in New York introduced it to the world market as the gem tanzanite, named after its country of origin.(3)  In 1996 and 1997, the mining of tanzanite boomed and the market was flooded, causing the price to drop drastically.  Then in 1998 terrible floods entered the mines and killed over one hundred miners.  As a result, the prices rose again and remain high.(4)  Another factor in the price of tanzanite is the increased popularity in recent years.

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Other Zoisite Minerals

Zoisite can be many colors other than the beautiful blues and purples of tanzanite.  It can be gray, yellowish brown, green, or pink.  Green zoisite is known as anyolite and is found in Austria.  Red or pink zoisite is known as thulite and is found in Norway, Italy, North Carolina and California.  Zoisite was named after an Austrian scientist, Baron Siegmund Zois van Edelstein.  Most non-tanzanite zoisites are used as ornamental stones or can be used as gems, but are usually cabochon cut, not faceted.(5)


Image taken from
http://mineral.galleries.com/
Minerals/Silicate/ZOISITE/ZOISITE.htm

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Gemstone Use

Tanzanite, like many gems, is priced by carat weight.  However, most importantly, tanzanite's price is determined by it's color.  The more blue tanzanite is costly because, due the a blue color axis oriented along the width rather than the length of the crystals, more of the stone goes to waste when it is cut to bring out the blue.  The blue is also more valuable because it resembles blue sapphire.  The purple stones tend to resemble amethyst, making them less valuable.  Tanzanite is a very delicate gem because of its low hardness and should not be used in everyday jewelry. Great care should be taken to ensure the stone will not be scratched or chipped.  Tanzanite should not be cleaned with an ultrasonic cleaner because it could shatter; instead, use lukewarm water with a mild detergent to wash it then rinse with water the same temperature.(2)  As mentioned earlier, tanzanite aquires its beautiful color by exposure to heat, therefore, all gem tanzanites are heat treated(6).  Synthetic tanzanites have been advertised at lower prices(7).  However, it is reported that tanzanite has not been synthesized(8), but rather that lab created minerals with the same look and optical properties of tanzanite do exist(7).  An excellent imitation was created in Russia, and this lab created gemstone is called fosterite.(6)


Image taken from:
http://www.gemhut.com/zoisite.htm

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References:

1. http://www.jewelrycentral.com/Target_Tanzanite.html

2. http://www.cwjewelers.com/stonetanz.htm

3. http://www.trustanmining.offc.com

4.  http://www.tradeshop.com/gems/zoisite.html

5. http://mineral.galleries.com/Minerals/Silicate/ZOISITE/ZOISITE.htm

6. http://www.bel-cg.com/articles/tanzanite.htm

7. http://www.africagems.com/articles/gemstonenews.html#Synthetic

8. http://www.devoredesigns.com/FactsandFallacies.htm

9.  http://www.webmineral.com/data/Zoisite.shtml

10.  http://www.britannica.com/eb/article?eu=80531&tocid=0

11.  http://www.gemhut.com/zoisite.htm

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This web page was created for Mineralogy at Emporia State University; under the instruction of Mrs. Susan W. Aber.  Please send questions or comments to Jessica Berg. This page was created on April 12, 2001.

copyright 2001 © Jessica Berg All rights reserved.