Block C is one of four principal mining concessions that divide the Merelani tanzanite deposit in northern Tanzania, situated at the foot of Mount Kilimanjaro in the Arusha Region. Covering the largest surface area of the four blocks — designated A, B, C, and D — it was for two decades the operational centrepiece of TanzaniteOne Ltd, a subsidiary of the South African mining group Richland Resources, and served as the primary source of large-volume tanzanite rough entering the international wholesale market. In 2020, the Tanzanian government nationalised the concession, transferring operational control to the state-owned Tanzania Minerals Audit Agency (TMAA), a move that reshaped the supply dynamics of the global tanzanite trade. Block C does not produce tanzanite of any inherently different quality from the adjacent blocks; gem characteristics are determined by individual crystal habit, growth conditions within specific pockets, and the skill of subsequent cutting rather than by concession boundaries.

The Merelani Deposit and the Block System

The Merelani Hills tanzanite deposit, discovered in 1967 by Manuel d'Souza, occupies a narrow metamorphic belt of graphitic gneisses and calc-silicate rocks associated with the Mozambique Belt — a Neoproterozoic orogenic system that runs the length of eastern Africa. Tanzanite, the blue-to-violet gem variety of the mineral zoisite coloured by vanadium and chromium, occurs in this deposit and nowhere else on Earth in commercially significant quantities, making the management of each concession a matter of considerable geopolitical and economic importance to Tanzania.

The Tanzanian government formalised the block system in the 1990s, dividing the deposit into four lettered concessions to regulate access and taxation. Blocks A and D were allocated to small-scale and artisanal miners, while Block B was held by a separate licence holder. Block C, by contrast, was granted to large-scale industrial operators, a status that enabled mechanised shaft sinking, geological mapping, and the kind of systematic ore extraction that smaller operations could not sustain.

TanzaniteOne and Industrial-Scale Operations

TanzaniteOne Ltd acquired the Block C licence in the early 2000s and invested substantially in underground infrastructure, including ventilated decline shafts reaching depths in excess of 800 metres. The company operated a vertically integrated model: rough tanzanite extracted from Block C was sorted, graded, and sold through a proprietary trading platform — the TanzaniteOne Trading Company — which sought to introduce greater price transparency and supply discipline to a market historically characterised by opacity and informal dealing.

At its peak, TanzaniteOne accounted for a significant share of global tanzanite rough supply, and its grading nomenclature — a colour-grade system using letter and number designations — gained some traction in the trade, though it was never universally adopted. The company also invested in marketing initiatives designed to raise consumer awareness of tanzanite as a distinct gem category, partly in response to concerns within the trade that tanzanite lacked the brand recognition of ruby, sapphire, and emerald despite its geological rarity.

Richland Resources, the parent company, faced persistent challenges common to single-asset mining operations: fluctuating rough prices, the high cost of deep underground extraction, and the structural difficulty of sustaining investor confidence in a gem whose value is not underpinned by commodity futures markets. The company was delisted from the London Stock Exchange's AIM market in 2019, shortly before the Tanzanian government moved to reclaim the concession.

Nationalisation and TMAA Oversight

The transfer of Block C to state control in 2020 was consistent with a broader policy direction pursued by the administration of President John Magufuli, which sought to increase Tanzanian state participation in the extractive sector across diamonds, gold, and coloured gemstones. The Tanzania Minerals Audit Agency, previously a regulatory and auditing body rather than a mining operator, assumed responsibility for the concession. The transition raised legitimate questions within the trade about continuity of production, maintenance of underground infrastructure, and the pace at which rough would reach the market.

In practice, the nationalisation did not immediately halt production, though output volumes and the regularity of supply to international buyers became less predictable in the years immediately following the transfer. Dealers and cutters who had previously purchased rough through TanzaniteOne's structured sales process found themselves navigating a less formalised procurement environment. Some trade observers noted that this uncertainty, combined with the existing informal activity in Blocks A and D, reinforced the already fragmented nature of tanzanite supply chains.

Gem Quality and the Absence of Block-Specific Characteristics

A persistent misconception in the retail trade holds that tanzanite from one block is inherently superior to that from another. This is not supported by gemmological evidence. The mineralising conditions within the Merelani deposit — the specific combination of temperature, pressure, fluid chemistry, and host-rock composition that produces strongly trichroic, vanadium-bearing zoisite — are functions of local geological pockets distributed across the entire deposit rather than of concession boundaries, which are administrative constructs.

Tanzanite from Block C has yielded exceptional stones, including crystals of several hundred carats in the rough, but comparable material has emerged from artisanal workings in Blocks A and D. The factors that determine gem quality in any given parcel are:

• Colour saturation and hue: the most prized stones display a vivid violetish blue to blue-violet in their primary face-up colour, with the secondary violet component visible in the pleochroic axes.
• Clarity: eye-clean to loupe-clean material commands significant premiums; tanzanite frequently contains growth tubes, fractures, and fluid inclusions that reduce clarity grades.
• Crystal size: tanzanite is notably size-sensitive in its colour; smaller stones often appear lighter and more lavender, while larger crystals more readily display the deep blue associated with top colour grades.
• Cutting quality: because tanzanite is strongly trichroic — appearing blue, violet, and burgundy along its three optical axes — the orientation of the cut relative to the crystal axes is critical to the face-up colour achieved.

Heat treatment, which converts the brownish component present in most rough tanzanite to the stable blue-violet seen in virtually all faceted stones on the market, is applied universally and is fully accepted by the trade and by major gemmological laboratories. This treatment is not specific to Block C material and does not vary by concession of origin.

Market Significance and Provenance

Because tanzanite is mined in a single deposit, country-of-origin certification — Tanzania — is straightforward and uncontested. Concession-level provenance (i.e., Block C specifically) has not historically been a driver of premium pricing in the way that, for example, Mogok or Kashmir origin commands premiums for ruby and sapphire. The trade has not developed a reliable analytical method for distinguishing Block C material from adjacent blocks, nor is there a commercial incentive to do so given the absence of quality differentiation.

What Block C's history does underscore is the vulnerability of a gem whose entire global supply originates from a single, geographically constrained deposit subject to the regulatory and political decisions of one government. The nationalisation of Block C serves as a case study in sovereign resource risk — a consideration that sophisticated buyers, dealers, and investors in tanzanite must weigh alongside the more familiar variables of colour, clarity, and cut.

Further Reading

• GIA Gem Encyclopedia: Tanzanite
• GIA: Tanzanite from the Merelani Hills
• AGTA Gemstone Encyclopedia: Tanzanite