Gem library
Tanzanite
The blue-violet of a single hill
Tanzanite is the blue to violet variety of zoisite, and the most extreme case of geographic rarity in gemmology: it comes from one hill eight kilometres long, in Tanzania. Discovered in 1967, it became within fifty years one of the best-selling coloured stones in the world.
- Hardness (Mohs)
- 6 – 7
- Density
- 3.10 – 3.38
- Crystal system
- Orthorhombic
- Refractive index
- 1.691 – 1.700
- Cleavage
- Perfect on {010}
Mineralogical identity
- Family
- Zoisite — silicate group (sorosilicates)
- Species
- Zoisite
- Variety
- Tanzanite
- Chemical formula
- Ca₂Al₃(SiO₄)₃(OH)
- Crystal system
- Orthorhombic
- Classification
- Semi-precious stone
- Rarity
- Rare
- Indicative price
- €150 – 900/ct
Etymology
Named in 1968 by Tiffany & Co. after Tanzania, the only country where it occurs. Its mineralogical name remains «blue zoisite».
Physical & optical properties
- Hardness (Mohs)
- 6 – 7
- Density
- 3.10 – 3.38
- Refractive index
- 1.691 – 1.700
- Birefringence
- 0.009
- Cleavage
- Perfect on {010} — one direction, the stone's weakness
- Fracture
- Uneven to conchoidal
- Lustre
- Vitreous
- Diaphaneity
- Transparent
- UV fluorescence
- None
- Pleochroism
- Distinct
- Colours
- blue, violet, indigo
Formation, geology & history
Formation and geology
Tanzanite formed roughly 585 million years ago during the Pan-African orogeny, when colliding cratons subjected calcium-rich sedimentary rocks to intense metamorphism. Its colour comes from traces of vanadium (V³⁺) trapped in the zoisite structure. The required coincidence — high-pressure metamorphism, calcic chemistry, available vanadium, locally folded tectonics — is so improbable that no second deposit has been found in sixty years of prospecting across East Africa. Geologists put the odds of the phenomenon repeating at roughly one in a million.
Heating, a universal practice
Straight out of the ground, tanzanite is usually brown, or a dull brownish violet. Gentle heating between 500 and 650 °C shifts the vanadium to another oxidation state and releases the blue-violet. The change is stable, permanent, and applied to virtually the entire production: naturally blue tanzanite is a collector's curiosity, not the market norm. The trichroism, on the other hand, is entirely natural: along different axes the same crystal appears blue, violet or burgundy. That is why the cutter chooses an orientation — blue for Western markets, violet for Asia — and deliberately sacrifices weight to get it.
A market under pressure
The Merelani deposit is estimated to run out between 2040 and 2060 at current rates. That known deadline sustains an investment narrative worth treating with caution: tanzanite remains abundant in the short term, and its prices have swung considerably. Since 2019 Tanzania has banned the export of unprocessed rough in order to build a local cutting industry, and a twenty-four-kilometre wall has been built around the mining zone to control smuggling. Trade colour grading runs from vB (violetish blue) to bV (bluish violet), with «AAA» and «Vivid» saturations commanding the highest prices.
A stone to wear with care
Tanzanite combines moderate hardness (6 to 7) with perfect cleavage in one direction. In practice, a knock on the corner of a set stone can split it cleanly, where a sapphire of the same size would merely chip. It is a superb gem in a pendant, in earrings or in an evening ring; it is a poor choice for a ring worn every day, especially on the dominant hand. A closed setting or covering claws reduce the risk appreciably.
How to recognise it
- Trichroism, its signatureTurn the stone slowly under white light: tanzanite shifts from blue to violet, and usually flashes burgundy along a third axis. No glass imitation reproduces this triple play.
- A blue that leans violetA blue sapphire stays blue; tanzanite almost always keeps a violet component, most visible under incandescent light, where it strengthens further. A frankly pure blue should suggest sapphire or spinel instead.
- Density and cleavageAt 3.10-3.38 it sinks in heavy liquids where quartz floats. Under the loupe, look on the girdle for incipient cleavage planes, highly characteristic: fine straight parallel lines rather than irregular breaks.
Imitations, treatments & confusions
- Cobalt glass and «tanzanique»Blue-violet glass is common in costume jewellery. It shows no pleochroism, reveals round bubbles under the loupe and feels warm to the touch. The trade name «tanzanique» always means glass, never tanzanite.
- Cubic zirconia and synthetic forsteriteTinted zirconia is too bright and too dispersive: it throws rainbow fire that tanzanite never produces. Synthetic forsterite, created specifically to imitate it, comes close in colour but stays greener and is far less dense.
- Iolite sold as tanzaniteIolite is blue-violet and pleochroic, which makes it credible to the eye. It is greyer, darker, less dense (2.6) and worth ten to fifty times less. A surprisingly low price points to iolite.
Deposits & origins
A single deposit on Earth: the Merelani Hills, Simanjiro district, Manyara region, at the foot of Kilimanjaro in Tanzania. Four mining blocks (A to D) along a strip eight kilometres by two — that is the entire world supply.
Care & precautions
- Warm water, mild soap, soft cloth. No stiff brush on set stones: cleavage dislikes pressure.
- Never ultrasonic or steam cleaning: the vibrations exploit the cleavage plane and crack the stone.
- Avoid sudden temperature changes, bleach, acetone and nail varnish; take rings off before sport and housework.
- Store in a lined compartment, away from sapphires, topaz and diamonds, which would scratch it.
Traditions & crystal healing
- Chakras
- Third eye · throat · crown
- Zodiac signs
- Sagittarius · Gemini
- Element
- Air · Ether
- Month
- December
Too recent to carry an ancient tradition, tanzanite has been assigned by contemporary crystal healing the role of a stone of transformation: it is said to accompany life changes, open intuition and connect speech to thought by acting on the throat and the third eye. The Maasai, on whose land it was found, sometimes give it to new mothers as a birth stone.
The virtues described here belong to tradition and cultural heritage. They are in no way medical advice and never replace a consultation.
Frequently asked questions
Is tanzanite rarer than diamond?
Geologically, yes: it comes from a single eight-kilometre deposit, while diamond is mined on four continents. It is nonetheless cheaper, because demand and prestige count as much as rarity in setting a price.
Is all tanzanite heated?
Almost all of it, and there is nothing fraudulent about that: heating is a universal, stable practice accepted by every laboratory. It simply has to be disclosed. The rare naturally blue stones sell at a significant premium.
Can tanzanite be worn every day?
In a pendant or earrings, without concern. In an everyday ring it is inadvisable: perfect cleavage makes it vulnerable to knocks. Choose a protective setting and remove the ring for manual work.