by Erum Ali Qureshi
Popular for its intense deep violet to blue color, Tanzanite is actually the commercial name given to the mineral Zoisite. This beautiful gem is increasingly being used in fine jewelry across the globe instead of the much-used and hugely popular blue sapphire . Here we take a look at the properties and characteristics that distinguish it from other similar looking stones like iolite and blue sapphire.
Popular for its intense deep violetto blue color, Tanzanite stone is actually the commercial name given to themineral
Zoisite. This beautiful gemis increasingly being used in finejewelry across the globe instead ofthe much-used and hugely popularblue sapphire . Here we take a look
at the properties andcharacteristics that distinguish it
from other similar looking stones
like iolite and blue sapphire.
The stone Tanzanite, named after itscountry of origin Tanzania actually occurs as the mineral Zoisite. Itschemical composition is hydrouscalcium Aluminium silicate. Zoisiteoccurs in many colors such as pink,brown, yellow, green, blue and a peculiar color described as ‘khaki’.Only the deep violet to blue coloredvariety is referred to and marketed as Tanzanite. All these shades of color are liable to turn to the favored blue on careful heattreatment and this is usuallycarried out.
Tanzanite as we know it was discovered only in the year 1967 inthe Meralanie hills of the Lelatemadistrict of Tanzania. Prior to this the only varieties of Zoisite that had some claim to being a gemmaterial were the massive pinkvariety known as thulite and thegreen chrome-rich material, which
occurs as matrix for large hexagonalcrystals of ruby found in Tanzania.This variety however is used as anornamental material and prized for its rich green-red color combination.
Much of traditional India considers
blue sapphire an astrologically
effective stone. Therefore its use
in modern fine jewelry is limited. People prefer to wear it only if it
suits their sun sign or on the
advice of an astrologer. This is one
reason why tanzanite is gaining
popularity in traditionally
motivated markets such as India. It
astrological significance and
its color matches closely with blue
sapphire sometimes even exceeding
its allure due to its (tanzanite’s)
All colors of Zoisite crystals areheated at temperatures up to 380degrees Celsius, as the stones tend
to disintegrate at highertemperatures. Therefore care must betaken while cleaning jewelry setwith Tanzanite in ultrasoniccleaners as they might fracturebadly therein. Since heat treatmentis universal, it has no effect onprice, and most finished gems ofthis variety are assumed to beheat-treated.
Tanzanite is often confused with asimilar looking gemstone called Iolite. However when viewedthrough a dichroscope the pleochroic colors
of iolite (blue, violet and paleyellow) distinguish it fromtanzanite whose pleochroic colorsinclude (violet, green and blue). Adichroscope is a small tube likegemological apparatus that is usedto view the colors of the ordinaryand extraordinary rays in coloredgemstones. (See box for thedefinition of
There are also other differencesbetween tanzanite and Iolite. for one, the specific gravity of tanzanite (3.15 to 3.38) is muchmore than that of iolite (2.57 to2.61) meaning that the heft of a stone purported to be tanzanite willbe considerably more than that of iolite. The refractive index too
shows a marked difference Tanzanite-1.69 to 1.70) as compared to iolite1.54 to 1.60). Of coursemeasuring these two properties(specific gravity and refractiveindex) requires gemological training and knowledge and is best left tothe experts. The information given here is solely for reference
A table of tanzanite simulants is
given below along with their
violet and pale yellow
usually seen simply as
almost colorless and deep
Refractive index- 1.54 to
Specific Gravity- 2.57 to
|Blue Glass||Blue glass
cannot be much of a
convincing simulant for
tanzanite, for one the
typical inclusions in glass
(gas bubbles, mold marks)
chips and breakages will
give it away. Also it will
not display Pleochroism.
(Yttrium Aluminium garnet-a
Specific Gravity- 4.6
(approximately) affording it
substantial heft as compared
to tanzanite. Also it will
not display Pleochroism.
to light grayish-blue.
Refractive index- 1.76 to
Specific Gravity- 3.80 to
Pleochroism, blue to pink.
Refractive index- 1.63 to
Specific Gravity- 3.2
There is no universally accepted method of grading colored gemstones. Tanzanite One, a major commercial player in the tanzanite market, through its no-profit subsidiary, The Tanzanite Foundation, has introduced its own color grading system. The new system’s color-grading scales divide tanzanite colors into a range of hues, between blue violet and violetblue.
Clarity grading in colored gemstones is based on the eye-clean standard, that is, a gem is considered
flawless if no inclusions are visible with the unaided eye(assuming 20×20 vision). TheGemological Institute of America classifies tanzanite as a Type I gemstone, meaning it is normally eye-flawless. Expect gems with eye-visible inclusions to be traded at deep discounts.
Pleochroism is a phenomenon
associated with colored
gemstones. What happens is
light reacts differently as
it enters a gemstone
creating two or three
different colors that can be
viewed when the gemstone is
viewed through a device
known as the dichroscope.
Some stones, for instance
untreated tanzanite roughs
display this property even
without using the
dichroscope. Simply rotating
the stone against a light
source will exhibit colors
in the different