TANZANITE
This variety of the mineral zoisite was not considered a gem material until
1967, when a
beautiful, rich, blue to purple blue, transparent variety was found in Tanzania
(hence tanzanite). The name Tanzanite was given to this stone by
Tiffany & Co. in 1969 as a trade name and has stuck ever since. It is
usually found in veins of granite schist with calcite, sulfur, quartz, and
green garnet. Tanzanite can possess a rich, sapphire blue color (possibly with some violet
red or greenish
yellow flashes). This lovely gem can cost many thousands of dollars per carat
today in larger sizes. But
one must be cautious. It is relatively soft, so we do not recommend tanzanite
for rings (unless
it's set in a very protected setting) or for everyday wear in which it would be
exposed to
knocks and other abuse. One must also be aware that a very inexpensive, dull,
brownish zoisite
can become a beautiful, expensive tanzanite after heat treatment.
TREATMENT: Commonly heat treated
(permanent color)
MOHS HARDNESS: 6.5 (though fairly durable, as a result of the heating process,
they become more brittle. It is recommended to avoid cleaning in an ultrasonic
cleaner)
CHEMICAL COMPOSITION: Ca 2 Al 3 Si 3 O 12 (OH)
REFRACTIVE INDEX: o=1.685-1.705, r=1.688-1.710, q=1.697-1.725
(The angle light is bent traveling through a gemstone)
DENSITY (SPECIFIC GRAVITY) : 3.15-3.38
(The number of times heavier a gemstone is compared to water)
