Tanzanite Mining Blocks & Locales

What’s all this talk about Tanzanite “Blocks”?

Tanzanite, the delightful blue-purple member of the Zoisite family, has taken the world by storm in the last three decades. Sales of tanzanite have skyrocketed in recent years, exceeding sales of all other precious gems, except Sapphire. It is mined only in the East African area of Tanzania known as Merelani.

The tanzanite deposits are found in metamorphic rocks, marbles, and schists that belong to the Mozambique Belt (Rift Valley). The deposits run through the low hills of Merelani that rise from the Sanya plains near Mount Kilimanjiro. The deposit line periodically folds over itself creating concentrated pockets of tanzanite. These pockets provide gem miners with the richest pickings of these popular green, pink, ultramarine and purple-blue gemstones.

The tanzanite mining area has been divided into four sections called “blocks.” These blocks are lettered A, B, C and D. The different blocks together total about 20 square kilometers and have been allotted to different mining concerns.

A gem quality tanzanite crystal. Typically found in concentrated folds along the mineshafts.
The Tanzanian Ministry of Mines allotted Blocks B and D to small-scale local miners to create local Tanzanian entrepreneurship. Together they employ approximately 10,000 miners across 750 shafts and mines. Block C was allotted to the big South African company, African Gem Resources (AFGEM) in 1999. Block A was allotted to Kilimanjiro Mines Ltd. and is currently not in service.

Block D has the reputation for producing the rare and beautiful “AAA” quality tanzanites. Characterized by intense purples with glistening flashes of red, the finest D Block ts can be likened to an old French wine of a rarest vintage. Many jewelers will never see a D Block tanzanite, as the vast majority of high quality tanzanite on the market comes from the neighboring C Block. tanzanite from the C Block can be AAA quality. But, the vast majority of tanzanite seen in jeweler’s windows, while still gorgeous, is actually Block C tanzanite, not Block D.

As might be expected, the largest and most sophisticated techniques used in tanzanite mining take place in Block C. According to recent reports, there are three main shafts leading down from the surface—known as “Main”, “Bravo” and “Delta.” The “Main” shaft, located in the middle of the Block. “Bravo” is situated towards the southern boundary of the Block, while “Delta” is located toward the northern boundary of the Block. An interceptor shaft, known the “JW”, is cut into the “Main” block. Not only has “JW” produced the highest per ton yields (an amazing 60 Carats per ton) it will serve in the future as a ventilation shaft for the “Main,” ensuring abundant airflow.

Past estimations have given the deposits a life of 20 years. Mining is currently taking place at about 170m in depth, with proven deposits of tanzanite bearing rock going down to about 280m. But if the new reports, which suggest tanzanite bearing rock goes down as far as 400m, is correct the life of the deposits will be extended considerably.