Tanzanite: The Complete Guide
The blue-violet gem found commercially in just one small patch of hills in northern Tanzania. It was unknown until 1967, named by Tiffany & Co., and is nearly always heat-treated. Here is the full story.
Quick facts
From 1967 to today
Tanzanite is one of the youngest gems on the market. Its story involves a disputed discovery, a famous brand, and a wall around a mining town.
- 1967Blue crystals are found in the Mirerani hills. Credit is disputed: Tanzania recognizes Jumanne Mhero Ngoma, GIA credits Maasai herder Ali Juuyawatu, and prospector Manuel de Souza registers the first claims. About 90 more claims soon follow.
- 1968Tiffany & Co. becomes the main distributor and names the gem after Tanzania, partly to avoid the mineral name “blue zoisite.” A big publicity campaign follows.
- 1971Tanzania nationalizes the mines.
- 1990The mining area is divided into Blocks A, B, C, and D.
- 2002The American Gem Trade Association adds tanzanite as a December birthstone.
- 2003–2010Tanzania bans exports of unprocessed tanzanite to India (2003), then exports of rough over 1 gram (2010).
- March 2008Heavy rain floods Block B mines, killing at least 57 miners, one of several deadly floods at Merelani.
- 2017–2018The government builds a 24 km wall around the Mirerani mining area to curb smuggling. Reported revenue rises sharply after it opens.
- June 2020Small-scale miner Saniniu Laizer finds two record rough stones, 9.27 kg and about 5.1 kg, and sells them to the government for about US $3.3 million. He finds a third large stone that August.
How tanzanite formed
Tanzanite needed an unusual recipe: vanadium-rich ancient sediments, the heat and pressure of a mountain-building event, and fluids moving through cracks at just the right time.
- Carbon-rich sediments. Organic-rich sediments containing vanadium are laid down and later become graphite-bearing gneiss.
- Mountain building. The rocks are caught up in the Mozambique Belt, a zone of continental collision. Earlier high-grade metamorphism dates to around a billion years ago.
- Veins open. Pegmatitic veins and fractures cut through the altered graphitic gneiss.
- Tanzanite grows. About 585 million years ago, at about 650 °C and 5–6 kbar, zoisite colored by vanadium crystallizes in the veins with quartz, diopside, graphite, calcite, and grossular garnet.
- Veins stretch and break. Later deformation pulls the veins into sausage-like pieces called boudins, which is where miners find gem pockets.
Merelani: the only commercial source
All gem tanzanite on the market comes from a strip only a few kilometers long near Mount Kilimanjaro. Descriptions of its size vary, from about 7 km by 2 km to about 7 km².
Block A
Merelani HillsSmall-scale and local mining.
Block B
Merelani HillsLocal miners. Site of the deadly 2008 flood.
Block C
Merelani HillsThe largest block, run by a commercial operator (TanzaniteOne).
Block D
Merelani HillsArtisanal mining, with thousands of miners working more than 100 m underground.
Record breakers
- Largest tanzanite rough (Guinness): 9.27 kg (46,350 carats), found by Saniniu Laizer on 24 June 2020, alongside a second stone of 25,515 carats.
- A third Laizer stone of about 31,500 carats followed in August 2020.
- The deepest colors are usually found in cut stones over 5 carats.
Color, grading, and related gems
Top color
Intense violetish blue with red flashes is most prized. Saturation drives price most.
Blue vs. violet
Cutters choose between a more violet stone that saves weight and a bluer, smaller, pricier stone.
Clarity
A “Type I” gem that is usually eye-clean.
Unheated tanzanite
Rare. Natural rough is often brownish or bronze and shows purple, blue, and yellowish-green tones.
Green zoisite
Very rare green gem zoisite, sometimes marketed as “green tanzanite.” Strictly, only blue to violet zoisite is tanzanite.
Other zoisites
Pink thulite, and anyolite (green zoisite with ruby).
Cat’s-eye tanzanite
Rare stones with a chatoyant band of light.
Heated, coated, and imitation tanzanite
There is no synthetic tanzanite on the market, but there are plenty of imitations and some undisclosed coatings.
| Property | What it is | How to spot it |
|---|---|---|
| Heated tanzanite | The standard, stable treatment used on almost all stones | Accepted in the trade and has little effect on price |
| Coated tanzanite | Stones coated with cobalt-bearing or titanium films to deepen the color | Iridescence, wear on facet edges, and color too intense for the size. Large coated stones can be hard to detect. |
| Synthetic forsterite | A lab-grown mineral often sold as “tanzanite” | Much higher birefringence, so back facets look strongly doubled. Looks green through a Hanneman filter, while tanzanite looks pinkish-orange. |
| CZ, YAG, spinel, glass | Common imitations | Separated by refractive index, polariscope, and filters |
Myths vs. facts
Myth: Tanzanite comes out of the ground blue-violet.
Fact: Most rough is brownish, and nearly all gems are heated to reach the blue-violet color.
Myth: Heated tanzanite is fake or unstable.
Fact: Heating is routine, stable, and accepted in the trade.
Myth: There’s lab-grown tanzanite.
Fact: No synthetic tanzanite is on the market. “Lab tanzanite” is usually synthetic forsterite or another imitation.
Myth: Tanzanite is as tough as sapphire.
Fact: It is 6–7 on Mohs with perfect cleavage and only fair toughness.
Myth: Tanzanite has been known for centuries.
Fact: It was unknown to the gem trade until 1967.
Myth: “Green tanzanite” is tanzanite.
Fact: Strictly, it is green zoisite.
Myth: One person clearly discovered it.
Fact: Credit is disputed among Ngoma, Juuyawatu, and de Souza.
Care and display
Cleaning
Warm soapy water only. Ultrasonic and steam cleaners are never recommended.
Knocks
Perfect cleavage means a hard knock can split it. Protective settings are best for rings.
Heat and acids
Avoid high heat, sudden temperature changes, and hydrochloric or hydrofluoric acid.
Storage
Store it apart from harder gems like sapphire and diamond to avoid scratches.
Tanzanite in the shop
See all tanzanite
Frequently asked questions
Will tanzanite run out?
It has a single source, so supply is finite. One 2012 study projected the main commercial block could be depleted in about 30 years at the production rate at that time, but that is a projection, not a certainty.
Is tanzanite found anywhere else?
Not commercially. All gem tanzanite comes from the Merelani Hills.
Is tanzanite a birthstone?
Yes, for December, since 2002. It is also the 24th-anniversary gem.
Why does the color change as I turn it?
Trichroism: tanzanite shows different colors along its three crystal directions.
How do I clean it?
Warm soapy water only. No ultrasonic or steam.
How can I spot a fake?
Strong facet doubling and green in a Hanneman filter suggest forsterite. Iridescence and worn edges suggest a coating. A lab report is the surest answer.
Is tanzanite actually valuable?
Yes, but less than diamond, ruby, or sapphire. Fine tanzanite is a mid-priced gem: value depends mostly on color saturation (deep violet-blue is the most prized), size, and clarity. Because all of it comes from one small mining area in Tanzania, long-term supply is limited, but prices have swung up and down with marketing and production since its 1967 discovery.
Why was tanzanite banned?
Tanzanite has never been banned as a gem. In 2001 and 2002, news reports claimed tanzanite trading helped fund terrorist groups, and several major U.S. retailers temporarily stopped selling it. Later investigations did not substantiate the claims, and the trade adopted the “Tucson Tanzanite Protocols” in 2002 to document supply chains. Separately, Tanzania has at times restricted exports of uncut rough to encourage local cutting, and in 2018 it built a wall around the Mirerani mining area to curb smuggling.
Who should not wear tanzanite?
Anyone hard on their hands may want to skip tanzanite in an everyday ring. It is only 6 to 6.5 on the Mohs scale and has a cleavage direction, so a hard knock can chip or split it. It is better suited to earrings, pendants, and occasional-wear rings. Never steam-clean it or use an ultrasonic cleaner.
What makes tanzanite so special?
Three things: it comes from a single source on Earth (the Mirerani Hills of northern Tanzania); it is strongly trichroic, showing blue, violet, and a burgundy-bronze color in different directions; and its blue-violet color, caused by the element vanadium, is unlike any other common gem. See our natural tanzanite rough.
What is the luster of tanzanite?
Vitreous (glassy). Polished tanzanite has a bright glassy shine; rough crystals can look duller on weathered surfaces.
Is tanzanite rarer than diamond?
By geography, yes: diamond is mined on six continents, while gem tanzanite is known from one small area in Tanzania. The popular claim that it is “1,000 times rarer than diamonds” is a marketing slogan, not a measured figure. More in What Is 1,000 Times Rarer Than a Diamond?
Keep reading
Sources and further reading
- GIA: Tanzanite: properties, discovery credit, source
- GIA: Tanzanite history and lore: claims, Tiffany
- GIA: Tanzanite quality factors
- GIA: Tanzanite care and cleaning
- Wikipedia: Tanzanite: blocks, export rules, birthstone
- Wikipedia: Jumanne Mhero Ngoma
- American Gem Society: Tanzanite history
- Lotus Gemology: The tanzanite mines of Merelani
- Stellenbosch University thesis: Merelani geology and age
- Tanzania Journal of Science: Merelani host rocks
- JCK: 2008 Merelani floods
- Africanews: The Mirerani wall
- Guinness World Records: Largest tanzanite
- GIA (2008): Coated tanzanite (PDF)
- Gemology Project: Synthetic forsterite
- JCK: Natural-color tanzanite
- International Gem Society: Tanzanite
Heating temperatures, the mining area’s size, and who discovered tanzanite differ between sources. This page gives the range or presents each claim.