Tanzanite: Properties, Origin, Treatment and Value

Tanzanite is the blue-violet variety of zoisite, a calcium aluminium silicate that is otherwise an unremarkable rock-forming mineral, and it comes from one place: a few square kilometres of graphitic gneiss in the Merelani Hills near Arusha in northern Tanzania. It was found in 1967, named by Tiffany in 1968 after the country, and marketed from the first day as the stone that exists nowhere else. That is true. It is also nearly always heated, it is fragile, and its market is unlike that of any stone in this journal, because every fact about it fits on one hill.

What it is

Zoisite is Ca₂Al₃(SiO₄)₃(OH), an orthorhombic sorosilicate. Hardness 6 to 7, which is soft for a gem, made worse by a perfect cleavage in one direction and a brittleness that makes tanzanite the most easily damaged of the stones a jeweller sets regularly. Refractive index 1.691 to 1.700, birefringence around 0.010, specific gravity 3.35. The trichroism is strong and defines the stone: blue down one axis, violet down another, and down the third a burgundy or brown that, in most rough, dominates the crystal and has to be removed. The cutter chooses which of the first two to put face-up, and the choice is commercial: the blue orientation is prized and usually costs weight, the violet orientation keeps more of the crystal and sells for less.

Where the colour comes from

Vanadium, V³⁺, substituting for aluminium. In the crystal as it grows the vanadium also sits in a state that gives the brown component, and heating to around five or six hundred degrees converts it, removing the brown and leaving the blue and violet. The treatment does nothing else: it does not heal fractures, does not add colour that was not latent, and leaves no trace a laboratory can read. Some crystals were heated by the earth itself, in the metamorphism that made them, and came out blue; they are rare and command attention from buyers who want the word unheated on paper, though the laboratory can only say that heating cannot be determined. There is no fluorescence, and the depth of the colour depends on vanadium concentration and on the cutter’s orientation.

Where it comes from

The Merelani Hills, in the Manyara region south-east of Arusha, in the same graphitic gneiss belt that produces tsavorite a few kilometres away. The deposit is divided into blocks worked by a large operator and by small-scale miners, and the trade’s block names circulate as if they meant quality; they mean location. Zoisite occurs in many places and the blue-violet gem variety in none of them, so origin is never in question and a laboratory does not need to state it. The single source is also the single risk: the deposit is finite, production varies with politics and capital, and the marketing around scarcity has been running since the 1970s while the mine has kept producing.

How it is judged

Colour first, on the three axes, with the market preferring a saturated blue of medium to medium-dark tone and pricing violet lower. Saturation rises with size, because the colour is tone-dependent and small stones read pale, so a fine tanzanite is nearly always a large one; below a few carats the stone rarely shows what it can do. Then the orientation, which a buyer can read by tilting: a blue-oriented stone shows violet from the side, a violet-oriented one shows blue. Then clarity, where tanzanite is usually clean and included stones are discounted. Then cut, which should be deep enough to hold colour and protected enough to survive the cleavage. Then size, and the per-carat curve climbs gently by the standards of the trade until the very large saturated stones, which are priced as objects.

Treatments, and what to demand on paper

Heat is the condition of the species. Nearly every tanzanite has been heated to remove brown, the treatment is stable and undetectable, and laboratories report it as a standard assumption; a report saying no indications of heating means only that none could be found, and the market pays little more for it. Coatings appear on poor goods to deepen colour and are removed by ordinary wear. Synthetic tanzanite does not exist commercially, but forsterite and glass are sold as tanzanite in market towns, and a report confirms identity. The unheated premium that governs corundum does not apply here, and a buyer should not pay for it.

The market

Large, commercial, and marketed harder than any coloured stone except diamond. Tanzanite is sold on its single origin, its story and its colour, and it sells in volume through channels that price it as jewellery material. Fine large saturated stones with good orientation command real prices and are beautiful. As a store of value the stone is poor: the mine produces, the material is heated as standard, the resale market is thin, and the honest accounting for stones as assets applies with the spread made wider by the marketing. Buy it for the colour and expect nothing afterwards.

Care

The most demanding stone in this journal. Hardness 6 to 7 scratches in a pocket; the cleavage splits under a knock; thermal shock from a hot tap or a steamer can crack it; ultrasonic cleaning can shatter it. Tanzanite belongs in earrings and pendants, or in a ring with a protective setting worn on occasions, cleaned with warm water and a soft cloth. Treated that way it keeps its colour indefinitely.

Where I stand

Tanzanite is not a stone I hold, and the theory says why: it is heated as a condition of existence, it is fragile, and it is sold on a story rather than on the paper. When a client asks, I say that a fine large stone is a beautiful thing to wear and a poor thing to hold, and I point them toward tsavorite from the next hill if they want an East African stone that meets every rule this office keeps. That is not a criticism of the stone. It is what the stone is.

Questions collectors ask

Is all tanzanite heated?

Very nearly all. Tanzanite comes out of the ground brownish or with a strong brown component from its third pleochroic colour, and gentle heating removes that and leaves blue-violet. The treatment is stable, undetectable and accepted as the standard condition of the stone. Naturally blue crystals, heated by the earth, exist and are rare.

Why is tanzanite found only in Tanzania?

Because the geological event that made it happened once. Vanadium-bearing zoisite formed in a small area of graphitic gneiss in the Merelani Hills near Arusha, in conditions no other known deposit reproduces. Zoisite occurs elsewhere; the blue-violet gem variety does not.

Is tanzanite a good investment?

As a store of value, no. It is a single-source stone marketed on scarcity, but the mine still produces, the material is heated as standard, and the resale market is thin. As a stone to wear with care, a fine saturated blue of size is beautiful and reasonably priced. Buy it for the colour and expect nothing from it afterwards.