Tanzanite

Why there is only one source

Tanzanite needs an unusual combination to form: vanadium in the rock, the right pressure and temperature during metamorphism, and then to survive for five hundred million years without being destroyed. That combination has been found in commercial quantity in exactly one place — a strip of hills about four kilometres long near Mount Kilimanjaro, in the Manyara region of northern Tanzania, worked since 1967.

Prospectors have looked hard for a second deposit and not found one. This is genuinely unusual: sapphire, ruby, emerald and garnet all come from many countries. Tanzanite comes from Merelani or it is not tanzanite, and any listing claiming another origin is either mistaken or selling something else.

The mining area is divided into blocks and worked at increasing depth. Fine large rough is scarcer each season, and the trade prices large clean stones accordingly — the price per carat steps up sharply rather than climbing smoothly.

Is tanzanite heat treated?

Almost always, and this is not the scandal it can be made to sound. Rough tanzanite from the ground is often brownish or khaki. Gentle heat, around 500 to 600 degrees Celsius, removes the brown component and leaves the blue-violet. The change is permanent and there is no care implication whatsoever.

The treatment is so close to universal that the trade prices tanzanite on the assumption it has been heated. And here is the part most sellers leave out: heating in tanzanite is not detectable by standard gemmological testing. The major laboratories do not state heated or unheated on a routine tanzanite report, because they cannot. Anyone charging you a premium for "unheated" tanzanite is charging you for something nobody can verify.

What matters more, practically, is that heating is all that is done. Tanzanite is not routinely oiled, filled, dyed or diffused. If someone offers you a tanzanite at a price that seems impossible, the risk is not treatment — it is that the stone is glass, synthetic forsterite, or a coated quartz.

Pleochroism: the thing no photograph shows

Tanzanite is strongly trichroic. The crystal absorbs light differently along its three optical axes, so the same stone shows blue down one direction, violet down another and, before heating, a brownish red down the third. Turn it in your hand and it shifts.

This is why cutting matters so much on tanzanite and why two stones of the same weight can differ so widely. A cutter orienting for maximum blue loses weight; a cutter orienting for maximum weight gets a more violet stone. Weight retention and colour are in direct conflict, and where a cutter landed between them is most of the price.

It also means tanzanite looks different under different lighting. It leans violet under incandescent light and bluer under daylight. Any honest listing shows it under both.

How tanzanite is graded, and what the letters mean

There is no official grading system for tanzanite colour. The "AAA", "AAAA" and "AAA+" you see everywhere are invented by sellers, mean nothing consistent between one shop and the next, and appear on no laboratory report.

What does mean something is the plain vocabulary: hue (is it blue, violetish blue, or bluish violet?), tone (how light or dark, usually described on a scale from very light to very dark) and saturation (how strong, from greyish to vivid). Fine tanzanite is a vivid, medium to medium-dark, violetish blue.

Clarity matters less than in a diamond because tanzanite is a Type I stone — it typically grows clean, so eye-visible inclusions are a genuine fault rather than a normal feature. Size matters a great deal: colour saturates with depth, so small stones are usually paler, and a vivid one-carat stone is proportionally rarer than a vivid five-carat.

What tanzanite costs

Colour first, size second, everything else after. As indicative retail ranges, in USD: commercial stones under a carat sit around $80 to $200 per carat; good violetish blue at one to three carats runs $250 to $600 per carat; and vivid, well-cut stones above five carats reach $600 to $1,200 per carat and beyond for exceptional colour.

Be sceptical of anything far below that. The Tanzanian Mining Commission publishes its own valuation schedule for royalty purposes, and the marketplace listings that undercut it by an order of magnitude are usually very pale, very included, or not tanzanite.

These are guide ranges, not quotes. Every stone is different and we price each one individually with the grading stated.

Is tanzanite hard enough to wear?

It depends entirely on what you set it into. Tanzanite is 6 to 7 on the Mohs scale, and more importantly it has perfect cleavage in one direction — meaning it can split cleanly along a plane if struck at the wrong angle.

That makes it an excellent pendant and earring stone, and a stone to think carefully about in a ring worn every day. A protective setting — bezel, halo, or a lower profile — matters more on tanzanite than on almost anything else. Never clean it in an ultrasonic bath, and keep it away from sudden temperature changes.

For a hard-wearing everyday ring in a comparable colour, sapphire at 9 is the durable answer and costs more. We will say so rather than sell you the wrong stone.

Tanzanite and sapphire, side by side

They can look nearly identical across a table, and they behave nothing alike. Rotate the stone: tanzanite shifts visibly between blue and violet, sapphire holds a near-constant blue. On the scale, sapphire feels heavier for its size — specific gravity around 4.00 against roughly 3.35. On a refractometer sapphire reads about 1.76 to 1.77 and tanzanite about 1.69.

On the wrist the difference is durability, and on the invoice the difference is usually price: fine sapphire costs several times fine tanzanite at the same size.

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