(Bloomberg) ? Tanzanite One Ltd. will sell tanzanite, a violet-blue gem mined in Tanzania and used in jewellery, at six viewings a year to selected customers.

Six customers, known as sightholders, will be chosen based on their ability to distribute tanzanite, the Hamilton, Bermuda-based company said yesterday in a Regulatory News Service statement.

"Sightholders will have the assurance of an exclusive and consistent supply of rough tanzanite," Tanzanite One said in the statement. The company said in September it planned to triple production by the end of this year.

Tanzanite One will sell gems in the same way as De Beers, the world's biggest seller of rough diamonds. Johannesburg-based De Beers holds ten viewings a year for sightholders who cut and polish stones. The US accounts for about 80 percent of tanzanite sales.

Shares in Tanzanite One were unchanged in London at 73 pence at 2.23 p.m. They have gained 64 percent in the past year, giving the company a market value of 51 million pounds ($96 million).

The company's processing plant will handle about 10,000 metric tons of ore a month, chief executive Mike Nunn said in a September 23 interview. A ton of ore contains about 60 carats of tanzanite, each selling for about $250.