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! 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
calcium aluminum silicate
Ca 2 Al 3 [O/OH/Si 4 /Si 2 O 7 ] + Sr+Cr+Fe
Crystal System:
blue; violet
transparent; translucent
elongated prismatic
uneven; brittle
Fracture Lustre:
vitreous; pearly
Specific Gravity
7.50 to 8.0
very poor
Refractive Index
o=1.692 to 1.693; e= 1.70
Optic Character
biaxial positive
0.030, per Schumann; 0.019, per Arem
strong trichroic; deep blue/purple/green
Absorption Spectrum
broad band in yellow, green; weak bands at 5280 and 4550; weak lines in the red
Aqua Filter
brownish-red in private collection specimen
Chelsea Filter
brownish-red in private collection specimen
soluble in acids
Thermal Traits
very sensitive to heat; avoid thermal shock; fuses easily to pebbly (blister-like) glass
heat improves color
hollow growth tubes; graphite platelets; actinolite, staurite, and diopside crystals (very rare)