Control issues

In April 2005, TanzaniteOne Mining announced it had acquired the mining licence to a significant portion of the tanzanite deposit known as ‘Block C.’ Since then, rough prices have increased steadily, as the company solidified its control of the market, which is believed to be 40 to 50 per cent of total output. Some of the rough goes through its official channels, which are made up mostly of American sightholders, but some is traded in the secondary market through smaller independents.

The mine currently has an estimated life of up to 20 years, producing about 2.2 million carats every year. Mining tanzanite nets the Tanzanian government approximately $20 million US annually, with most of the finished gems ending up in the U.S. market and sales totaling about $500 million US annually. In 2010, the Tanzanian government banned the export of rough bigger than five carats to spur the development of local cutting facilities and thus, boost the economy and potential for profit from a major natural resource. However, an estimated $400 million worth of tanzanite is cut in Jaipur, India. Rough is also being cut in Thailand, Germany, China, and, of course, Tanzania.

Riot of colour

The world’s largest faceted tanzanite is 737.81 carats, although the 242-carat ‘Queen of Kilimanjaro’ is one of the most famous. Set in a tiara, the stone is accented with 803 brilliant-cut tsavorite garnets and 913 brilliant-cut diamonds, and is part of the private collection of Michael Scott, who was the first chief executive officer (CEO) of Apple Computers.

Tanzanite in large sizes is readily available, says Alex Barcados of C.D. Barcados Co., in Toronto. “It is easier to find stones in deep rich colours in the larger sizes (10 carats and greater) than smaller ones of less than two carats. I find there is little or no size premium in the per-carat price for pieces over three carats. It used to be that a predominately blue face-up colour was the most sought-after, but the market seems to look equally for purplish to bluish colours, provided they are as vivid as possible.”

To date, there is no universally accepted method of grading coloured gemstones, although TanzaniteOne has introduced its own colour grading system, which divides tanzanite colours into hues ranging from blue-violet to violet-blue. In testing tanzanite at our lab, we have noticed that stones bigger than 10 carats are generally richer and deeper in colour.

Clarity grading in coloured gemstones is based on the eye-clean standard, that is, a gem is considered flawless if no inclusions are visible to the unaided eye. The Gemological Institute of America (GIA) classifies tanzanite as a Type I gemstone, meaning it is normally eye-flawless. We have found a few tanzanites with fractures. Like any other gem, this stone can be clarity-enhanced with oil, although it is a very rare practice.

Treatments and disclosure

Tanzanite is heat-treated in a 550 C to 700 C furnace to produce a range of hues between bluish-violet to violet-blue. Some stones found close to the surface in the early days of the discovery were gem-quality blue that required no heat treatment. Ideally, stones should not have any cracks or bubbles, as they can shatter the gem when they expand in size.

Heating is done in Tanzania and likely also in India. Lisa Elser of Custom Cut Gems travels to Tanzania to buy rough, but cuts it at her workshop in British Columbia. “Heating is usually done after cutting, although some miners use wood fires to heat the rough,” Elser explains. “It does not take very high heat to change tanzanite from its brown/yellow colour to blue/purple. Heating is assumed, so jewellers always have to disclose heat treatment at point of sale.” Since heat treatment is common, it has no effect on price of finished gems in sizes less than three carats.

More recently, undisclosed tanzanites coated with a thin layer of cobalt and other elements to improve durability began appearing on the market and in laboratories. The coating can be detected using X-ray fluorescence spectroscopy. It should also be noted that since this treatment is not permanent, it must be disclosed at the point of sale.