Intorduction
Tanzanite is a zoisite mineral that is fairly new to the jewellery market. It was discovered in the 1960’s in Africa. Tanzanite is now a popular blue/purple gemstone in America and has even been placed in the same category as ruby, sapphire, emerald and opal in some markets. Its tremendous popularity, along with hand mining limitations, flooding, and government challenges, has created a shortage of this beautiful mineral.

Physical Properties
Tanzanite has a hardness of 6.5. It is blue to purple in colour, with a white streak. The luster of tanzanite is vitreous and it is transparent to translucent. It has a specific gravity of 3.2 to 3.4. Tanzanite has good one directional cleavage and uneven to conchoidal fracture. Tanzanite occurs in several habits, including prismatic, striated and columnar.

Tanzanite is trichroic or when viewd in various orientations, different colour result. The intense blue-violet colour is caused by vanadium. In one direction it is blue, in another, purple, and in a third direction it appears bronze. These colours are more intense in larger pieces of tanzanite (over 10 carats). The smaller ones are usually more pale. This property makes tanzanite easy to identify because it is a rare property.

Tanzanite looks different in different lighting situations. Sunlight usually makes it look more purple. Light bulbs and candlelight also bring out the purple and can make the grayish ones look brown. Fluorescent lights tend to bring out the blue more, as do overcast skies.

Crystallography
Tanzanite is in the orthorhombic crystal system. The crystal class is 2/m²/m²/m, while its space group is Pnmc. It has common forms of {100}, {111}, and {001}.

Chemical Properties
Tanzanite is a hydrated calcium aluminium silicate mineral with a chemical formula of Ca₂Al₃Si₃O₁₂(OH). It is 18.76% calcium, 12.63% aluminium, 19.71% silicon, .24% hydrogen, and 48.67% oxygen. Tanzanite has as molecular weight of 427.38 gm.

Localities
Geologically, tanzanite is found in metamorphic rocks, pegmatite rocks, and hydrothermal veins. Geographically, tanzanite is found in only one place in the world, Merelani Hills of Tanzania around Mount Kilimanjaro. Specifically, in Merereani is in the Arusha province, which is located in the Simanjiro Disrict of Northern Tanzania. It is south of the Kikuletwa River and about ten miles from the Kilimanjaro International Airport. The mines are now at a depth of 140 meters and cover an area of over 350 meters underground. Crystal quartz, pyrite, and various geodes have also been found in the mines.

History
According to legend, tanzanite was first discovered by some Masai herders when they stumbled on some blue crystals lying with some other rocks. They reported that the crystals had become blue when lightning caused a fire that swept through the area and heated the zoisite, changing the brown colour of the crystals to the wonderful blue/purple. The first person to identify this blue zoisite was George Kruchik in the early 1960’s, who was given some samples that were supposedly blue sapphire. However, Manuel D’Souza is the person credited with discovering the deposits where it is now mined. He found them while prospecting for rubies. Shortly thereafter, the Tiffany Company in New York introduced it to the world market as the gem tanzanite, named after its country of origin. In 1996 and 1997, the mining of tanzanite boomed and the market was flooded, causing the price to drop drastically. Then in 1998 terrible floods entered the mines and killed over one hundred miners. As a result, the prices rose again and remained high. Another factor in the price of tanzanite is the increased popularity in recent years.

Blue’s Brothers
The synthetic forsterite could be misrepresented – accidentally or intentionally – as tanzanite, the blue to purple gem that has taken the trade by storm in the past few years. While buying and selling a tanzanite simulant is fine – the products are beautiful and in most cases more durable than tanzanite – you should know what the simulants are so you can tell the difference. And of course you must pass this information on to your customer, preferably in writing. Failing to do so could be chalked up as fraud.

Tanzanite simulants include laboratory-grown forsterite, corundum, garnet, spinel and glass. The emergence of these simulants raises some concern because it’s easy for disreputable sellers to mix natural tanzanite parcels with look-alike lab-grown simulants without disclosing that fact. But knowing the physical and optical characteristics of tanzanite and its simulants will help you avoid making a mistake.

Synthetic Forsterite
The new laboratory-grown material, first shown in the U.S. at the 1999 Tucson gem and mineral shows, comes from the Soviet Union. Forsterite is a form of the mineral olivine (peridot) and is considered rare in nature.

Testing for Tanzanite
The quick test involves the use of an Filter, invented and developed by Hanneman Gemmological Instruments.

The synthetic forsterite that looks similar to tanzanite turns a bright green with the Aqua Filter, Synthetic Forsterite exhibits a greenish body colour, while Tanzanite shows a pinkish-orange colour.

Only Tanzanite shows both this pinkish-orange colour in the filter and the correct dichroic colours.

Other tests useful for identification of tanzanite and its simulants are :-