Let's Talk Gemstones
By Edna B. Anthony, Gemologist
(Contact the author for permission to reproduce this
article in any form.)
P.0. BOX # 49371; Colorado Springs, Co. 80949-9371
E-MAIL eba@bwn.net
ZOISITE: a Sorosilicate
TANZANITE
Tanzanite! Baron Sigismund
Zois von Edelstein of Slovenia could never have imagined that a variety
of the mineral he discovered in 1805 in the Sau-Alp Mountains would cause
tremendous excitement in the jewelry industry more than a century later.
The name zoisite has replaced the name saualpite that he bestowed upon
this member of the epidote group of sorosilicates. The scholar-collector
was probably more interested in its chemical make-up and crystal structure.
In the sorosilicates, two SiO4 tetrahedra share a single apical oxygen
atom and form isolated, double tetrahedral groups. Such Si2O7 groups and
independent SiO4 tetrahedra link chains of AlO6 and AlO4(OH)2 octahedra
that share edges to form the monoclinic crystals characteristic of the
epidote group. Formation at higher temperatures in regionally metamorphosed
calcareous schists and shales and a twin-like doubling of the cell along
the A axis cause zoisite to develop in the orthorhombic crystal system.
Clinozoisite, its chemically identical twin, retains the characteristic
monoclinic crystal structure of the epidote group.
Three varieties of zoisite
are used as gemstones today. Thulite, the dense massive red or mottled
pink and white variety, found in manganese deposits in Norway, Austria,
Italy, western Australia, and in North Carolina, can be confused with rhodonite.
It and the massive green variety that often houses ruby crystals and inclusions
of dark hornblende are used for cabochons and carved ornamental objects.
However, it is transparent tanzanite that has vaulted to a "most desired"
status in the jewelry industry since its discovery in 1967 on the African
continent in Tanzania. According to Dr. J. Kourimsky in The Illustrated
Encyclopedia of Minerals and Rocks, an Indian tailor, Manuel d'Souza, found
a beautiful blue gem crystal southwest of Kilimanjaro near his hometown
of Arusha. Its color resembled an especially vivid sapphire. Speculation
that it was a very fine iolite (cordierite), an exceptionally large crystal
of dumortierite, or a new mineral was rampant. The crystal was sent to
the German Society for Precious Stones in Idar-Oberstein for cutting and
finishing. Experts there determined that it was not a new mineral, despite
its high content of strontium and low incidence of iron. It was, rather,
a variety of the mineral zoisite. Tiffany of New York exhibited a keen
interest in this "blue treasure of Africa" and named it tanzanite. With
a blitz of publicity, the new star in the realm of gemstones was launched.
Tanzanite develops in fissures
of igneous rocks as a secondary mineral through alteration of the minerals,
where the surrounding rock is composed. It is also found in contact metamorphic
zones of calcareous (calcium rich) schists and shales. The largest known
tanzanite crystal weighs 126 carats. Gems over 5 carats are rare indeed.
Most natural crystals are grayish or brownish blue. Dr. Joel Arem states
that "crystals are heated to about 700 degrees F to create a deep, intense
blue with violet dichroism."
Gem tanzanite has been found
only in Tanzania and Kenya. Few known deposits, primitive mining conditions,
natural disasters, uncertain controls, changing marketing practices, a
very active rumor mill, and the great demand for the gem has kept the situation
in a constant state of flux. What is known now is that the demand has exceeded
the present supply of quality material available.
Although synthetic tanzanite
has not been created, several simulants have appeared on the market. Doublets
composed of natural tanzanite crowns attached to glass pavilions, synthetic
spinel components cemented with colored glue, and paste imitations exist.
Cortanite and coranite are trademarked names for tanzanite-colored synthetic
corundum. Many unscrupulous dealers are offering these simulants globally
and also to especially vulnerable tourists in resort areas. Any tanzanite
jewelry should be purchased from known and trusted sources.
The appeal of fine tanzanite's
vibrant purplish-blue color has made it a very popular choice for rings,
despite its fragility and relative lack of hardness. A perfect plane of
cleavage makes it extremely vulnerable to knocks, and it should never be
subjected to ultrasonic cleaning. The gem is better suited for mounting
in pendants, pins, earrings, and other jewelry receiving less wear and
tear.
Gemstone Properties
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calcium aluminum silicate Ca 2 Al 3
[O/OH/Si 4 /Si 2 O 7 ] + Sr+Cr+Fe |
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o=1.692 to 1.693; e= 1.70 |
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0.030, per Schumann; 0.019,
per Arem |
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strong trichroic; deep
blue/purple/green |
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broad band in yellow,
green; weak bands at 5280 and 4550; weak lines in the red |
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brownish-red in private
collection specimen |
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brownish-red in private
collection specimen |
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very sensitive to heat;
avoid thermal shock; fuses easily to pebbly (blister-like) glass |
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hollow growth tubes; graphite
platelets; actinolite, staurite, and diopside crystals (very rare) |