Block A is the northernmost of four demarcated mining concessions within the Merelani Hills, situated in the Manyara Region of northern Tanzania at the foot of Mount Kilimanjaro. Together with Blocks B, C, and D, it forms the entirety of the world's only known commercial deposit of tanzanite (zoisite var. tanzanite), a gem discovered in 1967 and now among the most commercially significant coloured stones of the twentieth and twenty-first centuries. Block A is distinguished from its neighbours principally by its operational character: it has been worked predominantly by small-scale and artisanal miners rather than by large industrial concerns, and it has yielded some of the largest and finest gem-quality tanzanite crystals on record.

The Merelani Concession System

The four-block demarcation of the Merelani deposit was formalised by the Tanzanian government during the 1990s as part of broader efforts to regulate what had been a chaotic and often dangerous free-for-all at the site since commercial mining began in the late 1960s. The blocks were assigned distinct boundaries and, in principle, distinct ownership or licensing arrangements, though the practical administration of each has evolved considerably over the decades.

Block C, the largest concession, was granted to TanzaniteOne (later rebranded under the Richland Resources group) and became the site of the deposit's most mechanised, large-scale underground mining operation. Block D has been associated with smaller licensed operators. Block B has similarly seen a mix of licensed small-scale activity. Block A, by contrast, remained the domain of artisanal and small-scale miners (ASM), working through a network of informal shafts and tunnels that honeycomb the hillside. This operational distinction has had significant consequences for the character of material that reaches the market from Block A.

Geology and Crystal Character

The tanzanite deposit at Merelani is hosted within a sequence of Neoproterozoic metamorphic rocks — principally graphitic gneisses and calc-silicate granulites — that were subjected to intense regional metamorphism approximately 585 million years ago. The gem-bearing horizon is a relatively thin but laterally extensive zone of calcium-rich metasediments in which zoisite crystallised under conditions of elevated temperature and pressure. Vanadium and chromium substitution within the zoisite structure is responsible for tanzanite's celebrated trichroic character, shifting between blue-violet, red-violet, and yellow-green depending on the crystallographic axis viewed.

Crystals from across the Merelani deposit share this fundamental geology, but Block A has a documented history of producing unusually large individual crystals. The artisanal mining methods employed there — hand tools, manual shaft sinking, and careful extraction by miners with intimate knowledge of the pocket structures — may paradoxically favour the recovery of intact large crystals that mechanical mining equipment might fracture. Several of the largest tanzanite crystals ever recorded as gem-quality specimens are attributed to Block A workings, though precise provenance documentation at the individual-crystal level remains inconsistent in the artisanal sector.

Operational History and Conditions

Artisanal mining at Block A has proceeded under conditions that are, by any measure, hazardous. The shafts are typically narrow, hand-dug, and inadequately shored, and the Merelani Hills are subject to seasonal flooding that has caused fatal inundations on multiple documented occasions. The Tanzanian government has periodically attempted to formalise and improve safety standards across the artisanal sector, with mixed results. Licensing arrangements for individual miners and small cooperatives operating within Block A have shifted over the years, and the block has at various points been subject to disputes over rights and boundaries.

Despite these difficulties, Block A has remained continuously productive. The artisanal mining community at Merelani is substantial — estimates have placed the number of individual miners working the site at several thousand at peak periods — and Block A represents a significant portion of their collective livelihood. The material extracted flows through a network of local dealers and brokers in the nearby town of Arusha before entering the international coloured-stone trade.

Quality and Market Significance

Tanzanite from Block A is not categorically distinct in colour or optical properties from material mined in adjacent blocks; the deposit is geologically continuous, and the same gem-bearing horizon extends across all four concessions. What distinguishes Block A commercially is the size potential of its crystals and the informal, dealer-driven supply chain through which its production reaches the market.

Fine tanzanite is graded primarily on the depth and purity of its blue-violet colour, with the most prized stones exhibiting a saturated, velvety blue with a secondary violet component — a combination sometimes compared in trade parlance to the finest Ceylon sapphires, though the two are optically quite different. Stones of this quality from any part of Merelani command strong premiums, and large examples (above ten carats in finished weight) are considered genuinely rare. Block A's history of producing large crystals means that exceptional stones of this scale have a reasonable probability of Block A origin, though formal documentation is rarely available for artisanal production.

Heat treatment is near-universal for tanzanite across all blocks. Rough tanzanite as extracted is typically strongly trichroic with a dominant brownish or burgundy component; gentle heating to approximately 400–500 degrees Celsius drives off this component, leaving the blue-violet hue that the market prizes. This treatment is stable, undetectable by most standard gemmological testing, and universally accepted within the trade. The GIA, in its origin reports for tanzanite, may reference block provenance where this information has been reliably documented at the point of extraction, though such documentation is more readily available for large-scale operations such as Block C than for artisanal Block A production.

Provenance Documentation and Laboratory Reports

The question of block-level provenance within Merelani is of growing interest to the trade, partly as a function of broader consumer interest in supply-chain transparency and partly because certain buyers attach a premium to stones with verified origins. For Block C material sold through TanzaniteOne's controlled supply chain, documentation of mine origin was relatively straightforward. For Block A, the artisanal supply chain presents a more complex picture: stones may pass through multiple hands between the miner and the first formal dealer, and documentation of the specific shaft or even the specific block is not systematically maintained.

Major gemmological laboratories, including the GIA, can confirm that a stone originates from the Merelani deposit — the only known source of tanzanite — but sub-deposit block attribution is not routinely possible from the stone itself, as there are no reliably distinguishing inclusion types or trace-element signatures that differentiate Block A material from that of adjacent concessions. Block provenance, where stated on a laboratory report, therefore reflects information supplied at the point of submission rather than a conclusion derived from the stone's gemmological properties.

Conservation and Future Outlook

The Merelani deposit is finite, and estimates of remaining reserves have been a subject of ongoing debate within the industry. The Tanzanian government has expressed interest in extending the productive life of the deposit through more systematic mining methods, and there have been periodic proposals to restructure the artisanal sector within Block A. The long-term future of Block A as a distinct artisanal concession will depend on the interplay of government policy, the economics of small-scale mining, and the rate at which the accessible gem-bearing horizon is exhausted. For the present, Block A remains an active and significant contributor to global tanzanite supply, and its association with large, high-quality crystals ensures it a particular place in the history of this uniquely localised gem.

Further Reading

• GIA Gem Encyclopedia: Tanzanite
• GIA: Tanzanite Origin Determination
• International Gem Society: Tanzanite