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Figure 1. (left) Corrosion tubes. Figure 2. (right) Lamellar twinning
Figure 1. (left) Corrosion tubes. Figure 2. (right) Lamellar twinning

Understanding inclusions: part 4 tanzanite

Compared to other gemstones, tanzanite is young. JUNE MACKENZIE discusses this popular gemstone that Tiffany & Co brought to market in the 1960s.

What is tanzanite, the beautiful blue to violet- blue gemstone? Tanzanite ranges in colour from pale blue to pale violet-blue for smaller gemstones and a more saturated colour of blue or violet-blue for those larger in size. It belongs to the epidote group and sub-group called zoisite.

Although opinions may vary, generally pure saturated-blue tanzanite is the most highly prized in Europe. This colour is usually found in larger gemstones exceeding 4.00 carats; however, it may be also found in small gemstones from 1.50 to 2.00 carats. The US market tends to prefer the pure, saturated blue to have a touch of violet in it.

Tanzanite was originally discovered by gypsum miner Ndugu Jumanne Ngoma in Tanzania – Merelani, Arusha – in an area called Kiteto in January 1967. Ngoma wasn’t credited with this find until 17 years later when the Government of Tanzania finally acknowledged his discovery of tanzanite and presented him with a Certificate of Proof. During that long wait, Ngoma experienced many disappointments as other people were incorrectly credited with his discovery and even once this was corrected, he did not become wealthy as a result of his discovery.

Tiffany & Co named this exquisitely-coloured gemstone after the country in which it was found and began marketing it in 1968.

Zoisite comes in various colours including pink, green and brown. Brown is the most dominant colour and transforms into tanzanite when heat treatment brings out various shades of blue and blue/violet. Although zoisite/tanzanite can be found naturally in these desirable colours, the majority of tanzanite has its colour produced this way. The change in colour from brown to blue commences at approximately 350 degrees Celsius and so gem-quality gemstones are exposed to temperatures varying from 350 degrees Celsius to approximately 500 degrees Celsius for time varying from seconds to minutes. It should be noted that raising the temperature beyond 600 degrees Celsius can have a disastrous effect, turning the gemstone white and causing fracturing.

Tanzanite’s refractive index is generally 1.691 to 1.700 and the specific gravity averages 3.33. It is optically biaxial and trichroic, showing three distinct colours in a dichroscope – these trichroic colours change to different colours after heat-treatment. Tanzanite is relatively soft with a hardness of 6 to 7 on Mohs Scale.

Not much study has been conducted into inclusions in tanzanite; however, as no high- temperature treatment is carried out on the gemstones, there are rarely any changes to existing inclusions. Graphite and corrosion tubes that may intersect might be present (Figure 1), as may growth tubes, which never intersect, lamellar twinning (Figure 2) and some liquid inclusions.

The most highly-prized colour of tanzanite might theoretically be a pure, saturated blue but demand really is in the eye of the beholder. Colour is always a personal choice so sales dialogue should target whichever colour customers prefer.

When discussing after-sales care, remind customers that tanzanite is relatively soft so care must be taken when wearing it. It is not advisable for the owner or jeweller to clean it in an ultrasonic cleaner.












ABOUT THE AUTHOR
June Mackenzie

Contributor •


June Mackenzie FGAA Dip DT, is a qualified gemmologist and gemmology teacher in NSW. She is the developer and presenter of the GAA Advanced Gemstone Inclusions course. For more information, visit: gem.org.au









Monday, 11 December, 2017 03:03pm
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