Tanzanite

Look at a tanzanite from the side and it is violet. Look down through the top and it turns a deep, velvety blue. That double colour is what made the stone famous, and it is why no two tanzanites look quite alike.

Tanzanite is not Namibian, and we will never call it that. It comes from one small area: the Merelani Hills in northern Tanzania, near Mount Kilimanjaro. We include it in this guide because it is one of the stones our visitors ask for most, and because a gem from only one place on earth deserves an honest introduction.

A young gem

Tanzanite is a newcomer. It was found in 1967, and the next year a New York jeweller launched it under the name of the country it came from. Within a few years it had become one of the best-known coloured stones in the world. Because it comes from a single deposit, every tanzanite in every shop traces back to the same hills.

Beliefs and legends

Tanzanite is too young for ancient legends, but it already has a story of its own, told across the gem trade. Lightning set the dry grass alight on the hills below Kilimanjaro, the story goes. When Maasai herders drove their cattle across the burnt ground, they found that brown crystals lying in the ashes had turned a brilliant violet-blue.

Part of the story is true. In 1967 it was a Maasai man who first noticed the crystals and showed them to a prospector. The fire is less likely: the blue crystals found that year had probably been warmed by heat deep in the earth, not by a grass fire.

What it is

Tanzanite is a blue-violet variety of a mineral called zoisite. Its crystals show different colours along different directions: blue, violet, and a third colour that varies from stone to stone. A good cutter chooses the angle that shows the colour the buyer will love most.

Treatments, honestly

Most tanzanite on the market has been gently heated. Most crystals come out of the ground a brownish colour, and heat turns them to the blue-violet the stone is known for. The change is permanent and accepted throughout the trade. We say so on every piece we sell.

How to judge a tanzanite

• Depth of colour sets the price. The most valued stones are a deep, saturated blue-violet. Paler lilac stones are lovely and far more affordable.
• Blue or violet? Some stones lean blue, some violet. Neither is "better"; choose what you like, and look at the stone in daylight as well as shop light.
• Expect it to be clean. Good tanzanite is usually free of inclusions you can see with the eye.
• Bigger stones show colour better. Small tanzanites can look pale; the deepest colours are usually seen in larger stones.

Caring for your tanzanite

Tanzanite is softer than most gems in this guide (6 to 7 on the Mohs scale). It is perfect for earrings and pendants. In a ring, choose a setting that protects the stone, and take it off for gardening, sport and housework. Clean it in lukewarm soapy water with a soft brush. Never use an ultrasonic or steam cleaner, and keep it away from sudden changes of temperature.

At a glance

Mineral	Zoisite
Hardness	6 – 7 Mohs
Colours	Blue to violet; deeper is rarer
From	Merelani Hills, Tanzania only
Treatments	Most are heated (permanent). Disclosed per stone
Care	Lukewarm soapy water, soft brush. No ultrasonic, no steam, no knocks