Block A Tanzanite
Block A Tanzanite
Mining concession, market myth, and the geology of the Merelani Hills
Block A tanzanite refers to tanzanite — the blue-violet gem variety of the calcium aluminium silicate mineral zoisite — extracted from the concession designated Block A within the Merelani Hills deposit of northern Tanzania, situated near the town of Arusha at the foot of Mount Kilimanjaro. The term surfaces periodically in trade discussions, occasionally accompanied by claims of superior colour or crystal size, yet it carries no recognised gemmological distinction. Tanzanite from all four concession blocks of Merelani shares an identical geological origin, and the qualities that determine a stone's value — hue, saturation, tone, clarity, and crystal size — are functions of individual crystal formation, not of the administrative boundary within which a crystal happened to be found.
The Merelani Deposit and Its Concession Structure
The Merelani Hills deposit remains the world's sole known commercial source of tanzanite, a circumstance that gives the locality exceptional importance in the coloured-gemstone trade. The deposit was discovered in 1967 by a Maasai tribesman, Manuel de Souza, and was subsequently brought into organised mining. The Tanzanian government divided the deposit into four lettered concessions — Blocks A, B, C, and D — allocated to different operators under licence from the Ministry of Minerals.
Block C, the largest concession, is operated by TanzaniteOne (now part of Richland Resources), which industrialised extraction from the late 1990s onward and became the dominant commercial supplier. Block D is worked by smaller-scale licensed miners. Blocks A and B have historically been associated with artisanal and small-scale mining, though the precise operational status of individual concessions has shifted over the decades as licences have changed hands and extraction methods have evolved.
The entire deposit occupies a narrow metamorphic belt roughly five kilometres long and two kilometres wide. Tanzanite crystallises in graphitic gneiss within this belt, formed under conditions of regional metamorphism approximately 585 million years ago. Because the mineralising hydrothermal event responsible for tanzanite formation was geologically continuous across the belt, the chemistry and crystal-growth environment do not change meaningfully at concession boundaries, which are legal and administrative constructs rather than geological ones.
Why Block A Acquired a Reputation
The association of Block A with exceptional material appears to derive from the early decades of tanzanite mining, when artisanal miners working the shallower, more accessible portions of the deposit — which included parts of Block A — occasionally recovered large, well-formed crystals of vivid blue-violet colour. Some of the most celebrated early tanzanite specimens, including stones that entered the collections of major auction houses and jewellery maisons in the late 1960s and 1970s, are understood to have come from the northern end of the deposit.
This historical association was amplified by trade marketing. Because tanzanite's entire supply originates from a single country, and because consumers familiar with ruby and sapphire provenance premiums (Mogok, Kashmir, Pailin) were accustomed to locality-based value distinctions, some dealers sought to introduce an analogous hierarchy within Merelani itself. Block A became the shorthand for this perceived hierarchy.
It is worth noting that the early surface and near-surface workings of any deposit tend to yield material that has been subject to natural weathering and secondary alteration, which can in some cases improve apparent colour. In tanzanite, the characteristic trichroic blue-violet hue is enhanced by heat treatment that converts brownish vanadium-bearing crystals to the prized blue-violet. Crystals closer to the surface may have experienced natural thermal events that partially replicated this effect. However, this observation applies to the near-surface zone across the deposit as a whole, not to Block A specifically.
Gemmological Equivalence Across Blocks
Tanzanite from any part of Merelani is chemically calcium aluminium silicate with the formula Ca2Al3(SiO4)3(OH), coloured principally by vanadium with contributions from chromium and titanium. Refractive indices (approximately 1.691–1.700), specific gravity (approximately 3.35), and pleochroism — the stone's celebrated trichroism showing blue, violet, and burgundy in different crystallographic directions — are constant across the deposit. No reputable gemmological laboratory, including the Gemological Institute of America (GIA), the Gübelin Gem Lab, or SSEF, issues reports that differentiate tanzanite by concession block. Origin reports for tanzanite confirm only that a stone originates from the Merelani deposit in Tanzania; sub-locality to block level is not a parameter that can be determined by standard gemmological testing.
The GIA's published research on tanzanite, including work appearing in Gems & Gemology, consistently treats Merelani as a single geological source. There is no published peer-reviewed evidence establishing systematic differences in colour distribution, inclusion character, or crystal morphology that correlate with concession boundaries.
Treatment Considerations
The overwhelming majority of tanzanite on the market — estimated by the trade at well above ninety per cent — has been heat-treated to convert the natural brownish or purplish-brown trichroic colour to the more commercially desirable blue-violet. This treatment is stable, undetectable by standard gemmological means in most cases, and universally accepted within the trade. It is applied to material from all concession blocks equally. Untreated tanzanite of fine blue-violet colour does exist and commands attention among specialist collectors, but its rarity is a function of crystal chemistry and geological chance, not of block provenance.
Provenance Certification and Market Reality
Block provenance for tanzanite is rarely, if ever, certified in a manner that would satisfy rigorous scrutiny. The chain of custody from artisanal miner to cutting centre to dealer is frequently fragmented, and material from different parts of the deposit is routinely mixed at the trading stage in Arusha. A dealer asserting Block A provenance for a parcel of tanzanite is making a claim that is, in almost all practical circumstances, unverifiable.
In the broader market, Block A provenance carries no documented price premium in published auction results or in the pricing structures of major gemmological laboratories. Value in tanzanite is assessed on the same criteria applied to all coloured gemstones: colour (with deeply saturated, vivid blue-violet of medium-dark tone being most prized), clarity (eye-clean stones commanding the highest prices), cut quality, and carat weight. Stones above five carats of fine colour are notably scarce and attract strong demand regardless of any claimed sub-locality.
Buyers encountering Block A claims in the marketplace are advised to evaluate the stone on its own merits — colour, clarity, cut, and weight as assessed by a reputable independent laboratory — rather than on the basis of a provenance assertion that cannot be substantiated and that carries no recognised premium in the established trade.