What is tanzanite? The stone, the source, the colour

Tanzanite is the violet-blue to blue-violet variety of the mineral zoisite, and it is mined commercially in one area of the world: the Merelani hills of northern Tanzania, which is where the name comes from. It was found there in 1967 and introduced by Tiffany & Co. in 1968, who named it after the country.

That is the short answer. The longer and more useful one is about colour, because tanzanite’s colour is not a fixed property of the stone in the way a sapphire’s is. It is partly a decision somebody made while cutting it, and partly a decision somebody made while heating it. Understanding those two decisions is most of what separates a buyer who knows what they are looking at from one who does not.

Where it comes from, and why that is the whole story

One deposit, and it is small enough to walk across.

The sole source of tanzanite is the Merelani Hills in the Lelatema Mountains, in the Manyara region of northern Tanzania. The workings sit in an area of about eight square miles near the base of Mount Kilimanjaro. Almost every other coloured stone you can name is found on more than one continent. This one is found in a patch of ground you could cross on foot in an afternoon.

That single origin is the reason the stone exists as a category at all. It is also the reason the trade around it is unusually concentrated, and worth understanding before you buy. The mining area is divided into blocks, and Block C is the largest, accounting for over 50% of the mining area and the only mechanised block. The rest is worked at much smaller scale.

For a buyer, the practical consequence is narrower than the romance suggests. A single origin does not tell you a stone was mined responsibly, or cut well, or described honestly. It tells you where the material started. Everything that determines whether a particular stone is worth its price happened after that, in a cutting house and a setting bench, and none of it is visible from the word “Tanzanian” on a label.

It is also why we say plainly on each piece where the workshop told us the stone came from, and where they did not. A house that sells one stone from one hill has no excuse for being vague about the hill.

Why it is blue, and why it did not start that way

Vanadium, and heat.

The blue in tanzanite comes from small amounts of vanadium within the zoisite mineral structure. Zoisite itself is a common enough mineral and usually not blue at all. The vanadium is the difference, and how that vanadium behaves is the difference between rough that looks brownish and a finished gem that looks violet.

Most tanzanite comes out of the ground carrying a brownish or yellow-green component alongside the blue. Heating changes it. When vanadium-bearing zoisite is heated to a temperature of 600 degrees Celsius for about 30 minutes, the oxidation state of the vanadium is changed, and the unwanted colour goes with it. Other sources put the working range slightly wider, at about 500 to 600 degrees Celsius.

Most gems are heat treated, which removes or reduces the brownish colour and maximises the blue and violet. The result is permanent, it does not fade, and it creates no additional care requirement. Nothing about it is a defect.

What matters is disclosure rather than the treatment itself. Heating is close to universal here, so a seller who presents an untreated stone as a distinguishing feature is making a claim that needs a laboratory report behind it. Ask for the report. If there is not one, treat the claim as marketing rather than fact, and price the stone as heated, because almost all of it is.

Three colours in one crystal

Tanzanite is trichroic, which means it shows a different colour down each of three directions through the crystal.

The three are blue, red-violet, and yellow-green when viewed through each of its three crystal axes. A cut stone is never one of those colours cleanly. It is a blend, and which colour dominates in the finished gem is decided by how the cutter orients the rough before the first facet is placed.

This has a consequence that surprises people. The blue axis is the short one in most crystals. A cutter chasing the strongest blue has to orient the stone to look down that axis, and doing so means cutting away more of the rough and finishing with a smaller gem. Orienting for violet keeps more weight. So a blue face-up tanzanite of a given size started as a noticeably larger piece of rough than a violet one of the same size, and it is priced accordingly.

Neither choice is wrong. A violet stone is not a failed blue one. But it does mean the question “is blue better than violet” has an answer that is about supply rather than beauty: blue is rarer at any given size, so it costs more, and whether you prefer it is a separate matter entirely.

Light changes the balance again. The same stone can read sapphire blue in natural light and violet under fluorescent light. This is worth knowing before you judge a stone in a shop and then find it looks different at home. Nothing has changed except the light, and both readings are the stone.

Size and colour are the same question

Small tanzanites are pale, and not because they are poor quality.

Colour saturation scales with the depth of stone that light passes through, so identical material looks weaker in a smaller gem. GIA is blunt about the threshold: stones generally have to be above five carats to reach a deep, saturated violetish blue or bluish violet, and accent stones under two carats are commonly a lighter colour.

This matters when comparing prices. A pale one-carat tanzanite and a vivid seven-carat one are not the same material at different sizes. The size is part of why the colour is there at all. The majority of tanzanite mined is light to medium purple and blue, and the stone’s reputation rests on the small number of intensely saturated ones.

Lighter stones are not a lesser thing to want. Pastel hues are more plentiful and more affordable, and have a subtle appeal of their own. They are a different purchase, and they should be priced like one. A cluster of small pale stones and one large vivid stone can cost the same and are not remotely the same object.

What the stone is physically

The numbers that matter are hardness, cleavage and density.

Property	Figure	Why it matters to you
Mineral species	Zoisite	Tanzanite is a variety name, not a species. A report will say zoisite
Mohs hardness	6 to 7, about 6.5	Softer than quartz at 7, so ordinary dust can scratch it over time
Cleavage	Perfect, in one direction	A sharp knock along that plane can split the stone rather than chip it
Trichroism	Blue, red-violet, yellow-green	The cutter chose which one you see
Refractive index	1.692 to 1.700	Lower than sapphire, so it returns light more softly
Specific gravity	3.35	A tanzanite feels lighter in the hand than a sapphire of the same size

Hardness and cleavage are separate ideas and people mix them up constantly. Hardness is resistance to scratching. Cleavage is a plane along which the crystal parts cleanly when struck. A stone can be hard and still fragile, and tanzanite is neither especially hard nor free of cleavage, which is why the setting matters as much as the stone.

How to judge one, in order

Colour first, then cut, then clarity, then size, and never one of them in isolation.

Factor	What actually matters
Colour	Saturation above all. Vivid, strongly coloured stones are the sought-after ones, and blue face-up is rarer than violet
Cut	The quality of the cut makes a large difference to brilliance. The stone should return light evenly across the whole face, not only at the edges
Clarity	Eye-visible inclusions reduce value, and are more obvious in lighter stones. Fractures matter more than the rest, because they are a durability problem as well as a clarity one
Carat	Inseparable from colour. Judge the two together, never in sequence

None of these four is scored on a standard scale, because no such scale exists for tanzanite. That is worth understanding properly before you compare two sellers’ stock: how tanzanite is graded, and why AAA means nothing.

Cushions, trillions and ovals are the most common shapes, for the same reason the colour question exists. Those outlines let a cutter follow the crystal rather than fight it, and a shape chosen to preserve colour will always beat a shape chosen to preserve weight.

Five things to ask before you pay

Every one of these has an answer, and a seller who cannot give it is telling you something.

What is the treatment, and is it on a report? Heating is normal and expected. The answer you want is “heated, and here is the report”. The answer that should worry you is “ours is untreated” with nothing to show for it, because most gems are heat treated and a genuine exception is a documented exception.

Is that colour the face-up colour? Ask to see the stone away from the display lighting, in daylight if you can. Tanzanite reads sapphire blue in natural light and violet under fluorescent light, and a shop lit to flatter the stock is doing exactly what it was designed to do.

What is the setting protecting? With perfect cleavage in one direction, the girdle and the corners are where a stone gets hurt. A bezel or a recessed setting is worth more here than it would be on a sapphire, and a high four-claw solitaire is the least protective thing you can put a tanzanite in.

Is the price about the colour or about the carats? These are separate questions that get sold as one. A large pale stone and a small vivid one can sit at the same price for opposite reasons, and knowing which you are buying is the difference between a considered purchase and a heavy one.

Who wrote the grade? If there is a letter on the ticket, ask which laboratory assigned it. There is no standard scale for this stone, so the honest answer is always “we did”, and a seller willing to say so is more trustworthy than one who implies otherwise.

What tanzanite is bad at

Being knocked, and being cleaned carelessly.

It ranks 6 to 7 on the Mohs scale, with fair to poor toughness and cleavage, a tendency to break when struck. GIA’s own answer to whether it is tough enough for rings is measured: not as tough as ruby and sapphire, so consider a setting that protects the stone, or choose tanzanite for a ring you do not wear every day. The Gem Society puts it more directly still, advising that you reserve these gems for occasional pieces or use protective settings.

For a stone worn on the hand daily, that question deserves a longer answer than a paragraph, and we have written one: tanzanite engagement rings, and the honest durability answer.

Cleaning is simpler and the rules are absolute. Warm water, a mild detergent and a soft brush, and never an ultrasonic or steam cleaner. Ultrasonic cleaning drives vibration through the stone and tanzanite has a cleavage plane for that vibration to find. Steam adds thermal shock to the same problem. Both are the standard back-of-shop tools, which is why a jeweller who does not ask what your stone is before cleaning it is the wrong jeweller.

Imitations, and the one thing that does not exist

Nobody grows tanzanite in a laboratory, but plenty of things are sold as it.

Synthetic tanzanites are unknown. However, many simulants do exist. The distinction is worth holding on to. A synthetic is the same material made by people, and for tanzanite there is no such thing. A simulant is a different material that looks similar: synthetic forsterite, coated glass, and blue-violet cubic zirconia all turn up under names designed to sound like the real stone.

Anything sold as tanzanite at a price that seems unrelated to the figures above deserves a report from a laboratory rather than a reassurance from a seller. The stone has no standard grade to hide behind, which cuts both ways: it makes honest description harder, and it makes a dishonest one easier.

If you want to see how we describe what we sell, including the fields where a workshop has not answered us yet, the pieces are here: every piece the maison sells.

How much is left, and why we will not tell you it is running out

There is a figure, and it is larger than the sales line suggests.

A 2018 study by Tanzania’s National Bureau of Statistics put total tanzanite reserves at 109 million carats, with Block C alone estimated at about 87.1 million carats. Those are estimates of material in the ground rather than of gem-quality material that will reach a market, and the two are very different numbers.

You will read constantly that tanzanite will be exhausted within a generation. It is the oldest argument this stone has, it is repeated by people with something to sell, and we have not found a gemmological source that supports it as stated. So we do not repeat it. What is true and checkable is the single origin, the small working area and the concentration of production, and those are interesting enough without a countdown attached. The full account of how one hillside came to hold the world’s only deposit is in why tanzanite is found in only one place.

Buy tanzanite because the colour does something no other stone does, and because you understand what you are paying for. Not because somebody told you the clock is running.

If what you are after is what the stone is taken to mean rather than what it is, that is a separate and much more recent story: the birthstone, the symbolism and the astrology.

Three companion pieces go deeper than this one does. For the formula and the crystallography behind the three colours, what tanzanite is made of. For why almost every stone has been through a furnace, is all tanzanite heat treated. And for how it stands against the two stones it is most often weighed against, tanzanite against sapphire and diamond.

More on the stone, its origin and how it is judged: the tanzanite guide.