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 violet to blue color, Tanzanite stone 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.
The stone Tanzanite, named after its country of origin Tanzania actually occurs as the mineral Zoisite. Its chemical composition is hydrous calcium Aluminium silicate. Zoisite occurs in many colors such as pink, brown, yellow, green, blue and a peculiar color described as ‘khaki’. Only the deep violet to blue colored variety is referred to and marketed as Tanzanite. All these shades of color are liable to turn to the favored blue on careful heat treatment and this is usually carried out.
Tanzanite as we know it was discovered only in the year 1967 in the Meralanie hills of the Lelatema district of Tanzania. Prior to this the only varieties of Zoisite that had some claim to being a gem material were the massive pink variety known as thulite and the green chrome-rich material, which occurs as matrix for large hexagonal crystals of ruby found in Tanzania. This variety however is used as an ornamental 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 has no astrological significance and its color matches closely with blue sapphire sometimes even exceeding its allure due to its (tanzanite’s) violet tinge.
All colors of Zoisite crystals are heated at temperatures up to 380 degrees Celsius, as the stones tend to disintegrate at higher temperatures. Therefore care must be taken while cleaning jewelry set with Tanzanite in ultrasonic cleaners as they might fracture badly therein. Since heat treatment is universal, it has no effect on price, and most finished gems of this variety are assumed to be heat-treated.
Tanzanite is often confused with a similar looking gemstone called Iolite. However when viewed through a dichroscope the pleochroic colors of iolite (blue, violet and pale yellow) distinguish it from tanzanite whose pleochroic colors include (violet, green and blue). A dichroscope is a small tube like gemological apparatus that is used to view the colors of the ordinary and extraordinary rays in colored gemstones. (See box for the definition of Pleochroism.)
There are also other differences between tanzanite and Iolite. for one, the specific gravity of tanzanite (3.15 to 3.38) is much more than that of iolite (2.57 to 2.61) meaning that the heft of a stone purported to be tanzanite will be 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 course measuring these two properties (specific gravity and refractive index) requires gemological training and knowledge and is best left to the experts. The information given here is solely for reference purposes.
A table of tanzanite simulants is given below along with their identifying features.
|Pleochroic colors- Blue, violet and pale yellow usually seen simply as almost colorless and deep blue.
Refractive index- 1.54 to 1.60
Specific Gravity- 2.57 to 2.61
|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.
|Blue YAG (Yttrium Aluminium garnet-a synthetic crystalline material)
|Refractive index- 1.83
Specific Gravity- 4.6 (approximately) affording it substantial heft as compared to tanzanite. Also it will not display Pleochroism.
|Purplish-blue synthetic corundum
|Pleochroism- purplish-blue to light grayish-blue.
Refractive index- 1.76 to 1.78
Specific Gravity- 3.80 to 4.05
|Synthetic blue forsterite
|Strong Pleochroism, blue to pink.
Refractive index- 1.63 to 1.67
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 violet blue.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). The Gemological 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 orientations.