Tanzanite is the blue to violet variety of the mineral zoisite, and it is mined commercially in only one place: a small area in the Merelani Hills of northern Tanzania. Nearly all tanzanite on the market has been heated to turn its natural brownish or mixed colors into blue and violet. It is softer than sapphire and has one direction of perfect cleavage, so it needs more care in jewelry.
What tanzanite is
Zoisite is a calcium aluminum silicate that occurs in green, pink, gray, brown and other colors. Opaque green zoisite with rubies inside, from Tanzania, is sold as a carving material called ruby in zoisite. Pink zoisite is called thulite. The blue to violet variety colored by vanadium is tanzanite.
Tanzanite crystallizes in the orthorhombic system, usually as striated prisms. It has a hardness of 6 to 7 on the Mohs hardness scale, a refractive index around 1.69-1.70 and a specific gravity near 3.35.
Discovery at Merelani in 1967
Blue zoisite crystals were found in the Merelani Hills, near Arusha and within sight of Mount Kilimanjaro, in 1967. Accounts credit a local prospector, Manuel d'Souza, with registering the first claims, although Maasai herders in the area are often said to have known the crystals before then. Initial reports suggested the material might be sapphire, but testing showed it was a new gem variety of zoisite.
Tiffany & Co. began marketing the gem in 1968 and named it tanzanite after the country, a name easier to sell than "blue zoisite." The marketing presented it as a gem found in only one place in the world, and that remains true of commercial production.
Geology of the Merelani deposit
Tanzanite occurs in a narrow belt of folded metamorphic rocks, including graphite-bearing gneiss and marble, a few kilometers long. Hydrothermal fluids moving through these rocks during an ancient episode of mountain building deposited zoisite in veins and pockets, with vanadium supplying the color. The rarity of this combination of rocks and fluids helps explain why no other commercial tanzanite deposit has been found. The page on metamorphic rocks and gemstones describes how such deposits form.
The Tanzanian government divided the Merelani mining area into blocks labeled A to D, with blocks worked by both small-scale miners and larger companies over the years. In 2018, the government built a wall around the mining area to control smuggling. Mining is underground, often in narrow, steep shafts that follow the folded layers.
Heat treatment
Most tanzanite comes out of the ground with a brownish, greenish or grayish component mixed with blue and violet. Heating to a few hundred degrees Celsius removes the brownish color and leaves the blue and violet. Some crystals found near the surface show blue color without laboratory heating, probably because of natural heating in the ground.
The result of heating is stable and permanent, and it cannot usually be detected, so the trade assumes tanzanite has been heated. Coatings to improve color have also been found on some stones, and coated tanzanite should be disclosed and treated with care, because the coating can wear off. The overview of gemstone treatments covers both heating and coatings.
Trichroism
Tanzanite is strongly trichroic: it shows three distinct colors when viewed along different crystal directions. Before heating, these colors are typically blue, violet to purple, and a yellowish or brownish tone. After heating, the yellowish-brown direction changes so that the stone shows mainly blue and violet.
Cutters must decide how to orient the stone. Placing the table to show the blue direction produces a bluer stone, but it often costs more weight. Placing it to show violet can save weight. The resulting gems range from violetish blue to bluish violet, and many stones show flashes of both colors as they move. Tanzanite also tends to look bluer under fluorescent or daylight and more violet under incandescent light. The page on pleochroism in gemstones explains this property.
Judging tanzanite quality
Color sets value. The most prized stones show a deep, saturated violetish blue to blue. Paler stones, often sold as lavender tanzanite, cost much less. Large stones tend to show deeper color, because the light travels a longer path through them, which is one reason fine color is easier to achieve above a few carats.
Tanzanite is usually eye-clean. GIA places it in its Type I clarity group, so visible inclusions lower value more than they would in emerald or tourmaline. Cut quality affects brightness, and many tanzanites are cut as ovals, cushions and trillions. For a general framework, see how colored gemstones are graded.
Look-alikes and imitations
Blue sapphire can look similar but is harder and has a higher refractive index. Iolite also shows strong pleochroism but has a lower refractive index and specific gravity. Synthetic forsterite, glass and coated quartz have been sold as tanzanite imitations. Synthetic forsterite looks very similar but has much higher birefringence, which a gemologist can detect.
Cleavage, durability and care
Tanzanite has one direction of perfect cleavage. A sharp knock along that direction can split the stone, and sudden temperature changes can cause fractures. It is also softer than quartz, so everyday dust can wear facet edges over time. The pages on cleavage in gemstones and gemstone durability explain these risks.
Tanzanite suits earrings, pendants and brooches well. In rings, protective settings such as bezels or halos help, and the stone should be removed for sports, gardening and hard work. Clean tanzanite with warm soapy water and a soft brush. Do not use ultrasonic or steam cleaners. Jewelers should remove the stone before using a torch for repairs.
December birthstone
In 2002, the American Gem Trade Association added tanzanite to the list of December birthstones, alongside zircon and turquoise. It was the first change to the modern American birthstone list since 1952.
Frequently asked questions
Is tanzanite only found in Tanzania?
Yes, for commercial purposes. All gem tanzanite on the market comes from the Merelani Hills area of northern Tanzania. Blue zoisite has been reported elsewhere, but not in commercial quantities.
Is all tanzanite heated?
Nearly all of it. Heating removes brownish tones and is permanent and undetectable in most stones, so the trade assumes tanzanite has been heated.
Can tanzanite be worn every day?
It can, with care. Tanzanite is softer than quartz and has perfect cleavage, so protective ring settings and removing it during hard work help prevent scratches and chips.
Why does tanzanite change between blue and violet?
Tanzanite is trichroic and shows different colors in different directions. Lighting also matters: daylight and fluorescent light bring out blue, while incandescent light brings out violet.