Block B is one of four designated mining concessions — Blocks A, B, C, and D — that together constitute the Merelani tanzanite deposit in the Lelatema Mountains of northern Tanzania, near the town of Arusha and in the shadow of Mount Kilimanjaro. Of the four blocks, Block B has historically been associated with the most industrialised and capitalised extraction operations, and for much of the early twenty-first century it was the principal domain of TanzaniteOne, the largest single mining entity ever to operate at Merelani. Although the concession boundaries carry administrative and commercial significance, they carry no gemmological significance: tanzanite crystals do not differ in chemistry, optical character, or quality by virtue of which block they were extracted from.

The Merelani Concession System

The Tanzanian government divided the Merelani deposit into four lettered blocks as a means of regulating access and licensing. Blocks A and C were historically allocated to small-scale artisanal miners operating under licences issued by the Ministry of Energy and Minerals, while Block D has been associated with mid-scale operations. Block B, by contrast, was designated for large-scale industrial mining, a classification that reflected both the capital requirements of deep mechanised extraction and the government's interest in maximising formal-sector revenue from what is Tanzania's most economically significant coloured gemstone.

The geology underlying all four blocks is continuous: tanzanite — the blue-violet gem-quality variety of the mineral zoisite — occurs within a narrow belt of graphitic gneisses and calc-silicate rocks that were metamorphosed under high-pressure, high-temperature conditions during the Mozambique Belt orogeny, approximately 585 million years ago. The deposit is estimated to extend to considerable depth, and Block B's industrial operators have pursued mining well below the surface workings accessible to artisanal diggers in adjacent concessions.

TanzaniteOne and Large-Scale Operations

The name most closely associated with Block B in the trade literature is TanzaniteOne, a company that held large-scale mining rights over the concession and was, at its peak, responsible for a substantial proportion of the world's formal tanzanite production. TanzaniteOne operated mechanised underground mining, employed several hundred workers, and maintained sorting and grading facilities on site. The company also invested in marketing infrastructure, including the creation of a grading nomenclature and promotional programmes aimed at positioning tanzanite as a mainstream fine gemstone rather than a niche collector's stone.

TanzaniteOne's tenure at Block B was not without controversy. The Tanzanian government, under successive administrations, expressed concern that the benefits of tanzanite mining were not accruing sufficiently to Tanzanian citizens and the national treasury. A series of regulatory reviews and ownership restructurings took place during the 2000s and into the 2010s, reflecting broader Tanzanian policy shifts toward greater state participation in extractive industries. These changes affected the licensing arrangements at Block B and altered the corporate structure of operations there, though large-scale mechanised mining has continued under various successor arrangements.

Production and Commercial Significance

Block B has contributed significant commercial volumes of tanzanite to the global market, particularly in the lower-to-mid quality ranges that dominate retail jewellery. Large-scale mechanised extraction recovers material across the full quality spectrum, from heavily included or poorly coloured rough suitable only for calibrated commercial goods, through to fine and occasionally exceptional crystals that reach auction and high-end retail. The sorting and grading operations associated with Block B have historically fed both the Arusha cutting industry and the export trade to cutting centres in Jaipur, Bangkok, and elsewhere.

It is worth noting that the volume of material attributed to Block B in trade documentation is not always reliable. Tanzanite from artisanal workings in adjacent blocks — and from informal or undocumented digging — frequently enters the supply chain without clear provenance, and the Merelani deposit as a whole is treated as a single origin by gemmological laboratories. No major laboratory issues origin reports distinguishing Block B material from Block C or Block D material, because no analytical technique currently exists to make such a distinction.

Gemmological Characteristics

Tanzanite from Block B shares all the defining properties of the species regardless of concession. It is a calcium aluminium sorosilicate belonging to the zoisite group, with the chemical formula Ca₂Al₃(SiO₄)(Si₂O₇)O(OH). Its celebrated trichroism — displaying blue, violet, and burgundy or red-brown axes under polarised light — arises from vanadium and, to a lesser extent, chromium substitution within the crystal structure. The refractive indices are approximately 1.691–1.700, with a birefringence of 0.008–0.013, and the specific gravity is approximately 3.35. Tanzanite is relatively soft for a fine gemstone at 6 to 6.5 on the Mohs scale, with pronounced cleavage in one direction, making cutting and setting demands significant considerations.

Virtually all tanzanite reaching the market — from Block B and from every other source within Merelani — has been heat-treated. Rough tanzanite as extracted is typically brownish or tawny in colour due to the presence of the oxidised form of vanadium; gentle heating to approximately 400–500 degrees Celsius drives off the brown component and stabilises the blue-violet hue that the market prizes. This treatment is universally accepted, essentially undetectable by any current analytical method, and considered a standard part of tanzanite production rather than an enhancement requiring disclosure in the same manner as, for example, beryllium diffusion in sapphire.

Regulatory and Ethical Context

Tanzania has progressively tightened export regulations on rough tanzanite, with legislation in recent years requiring that a greater proportion of rough be cut and polished within Tanzania before export. These measures are intended to develop the domestic value-added industry and retain more economic benefit within the country. Block B's large-scale operations have been subject to these regulations, and the interplay between industrial mining interests, government policy, and the artisanal sector in adjacent blocks has made Merelani one of the more closely watched gemstone-producing regions in Africa from a supply-chain governance perspective.

Buyers and dealers sourcing tanzanite should be aware that formal provenance documentation to the concession level is rarely available in practice, and that claims of specific block origin — including Block B — cannot be independently verified by gemmological means. The responsible trade standard is to confirm Tanzanian origin and to ensure that material has passed through documented export channels consistent with Tanzanian law.

Further Reading

• GIA Gem Encyclopedia: Tanzanite
• GIA: Tanzanite Mining at Merelani
• ICA: Tanzanite