Tanzanite

Look at a tanzanite from the side and it is violet. Look down through the top and it turns a deep, velvety blue. On a summer evening it is the colour of the lagoon just after the sun has gone.

Tanzanite is not South African, and we will never call it that. Every one comes from a small area of hills at Merelani, in northern Tanzania, near Mount Kilimanjaro. It is one of the two stones we are best known for, alongside diamond, and a gem from only one place on earth deserves an honest introduction.

A young gem

Tanzanite was found in 1967. The next year Tiffany & Co. launched it, named after the country it came from, and within a few years it was one of the best-known coloured stones in the world. Because it comes from a single deposit of about seven square kilometres, every tanzanite in every shop traces back to the same hills.

Beliefs and legends

Tanzanite is too young for ancient legends, but the gem trade already tells a story about it. Lightning, the story goes, set fire to the dry grass on the hills of northern Tanzania, and herders crossing the burnt ground found that brown crystals in the ashes had turned violet-blue.

The part we know is true: in 1967 a Maasai herdsman found the crystals and showed them to a prospector. The fire is less likely. The first blue crystals had probably been heated naturally, deep in the earth.

What it is

Tanzanite is a blue-violet variety of a mineral called zoisite, coloured by a trace of vanadium. Its crystals show different colours in 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 comes out of the ground a brownish colour, and gentle heating turns it to the blue-violet the stone is known for. The change is permanent, and laboratories often cannot tell whether a stone has been heated, so the trade treats every tanzanite as heated. So do we. Heating is standard practice and is not listed piece by piece; if a stone has had any other treatment, we tell you.

Our tanzanite certificate

Many visitors ask how they can be sure of what they are buying. Ask whether the piece you are looking at comes with a Knysna Jewellers tanzanite certificate, our own in-house certificate for the stone.

How to judge a tanzanite

• Depth of colour sets the price. The most valued stones are an intense violetish blue. Paler lilac stones are far more common and far more affordable.
• Blue or violet? Cutting for violet wastes less of the crystal, which is one reason violet stones are more plentiful; the trade pays more for blue. Choose the colour you love.
• Light changes it. Cool daylight brings out the blue, warm light the violet. Look at it in both.
• Expect it to be clean. Most tanzanite sold for jewellery is clean to the eye.
• Bigger stones show colour better. The deepest colours are usually seen in larger stones; small ones can look pale.

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