Most gemstones show up in a handful of countries. Sapphire comes from Sri Lanka, Madagascar, Australia, Montana. Diamonds come from a dozen places. Tanzanite comes from one hillside in Tanzania, and nowhere else has ever produced it. Every tanzanite that’s ever been cut, cheap or expensive, came out of a strip of ground about four miles long and a mile wide.
A stone 585 million years old
Tanzanite is the blue-to-violet form of zoisite, a mineral that’s been known to science for a long time in duller, browner varieties. What makes it blue is trace vanadium locked into the crystal structure. The deposit formed around 585 million years ago, during a period of serious mountain-building heat near what’s now Kilimanjaro, in the folded hinge zones of buried rock layers. That’s an oddly specific set of conditions, and as far as anyone’s found, it happened in exactly this combination exactly once. That’s the real reason tanzanite is single-locality: not that nobody’s looked elsewhere, but that the geology behind it seems to be genuinely one of a kind.
Two discoveries, four months apart
Officially, credit goes to Jumanne Mhero Ngoma, who found blue crystals weathering out of the ground near Mererani in January 1967. Tanzania’s first president, Julius Nyerere, later recognized him for it and awarded him a scientific discovery certificate. Then in July of that same year, a Goan tailor and part-time gold prospector named Manuel d’Souza found similar crystals nearby and staked claims of his own, convinced at first he’d found sapphire. Gemological labs eventually sorted out what it actually was: gem-quality blue zoisite, a form nobody had seen before in transparent, richly colored crystal. Within a few years, dozens more claims went up across the same district.
How Tiffany saved it from a bad name
“Blue zoisite” was never going to sell in a jewelry case. Say it out loud and it lands uncomfortably close to “suicide,” which is reportedly exactly what worried Tiffany & Co. when the company picked up the stone as its main distributor. So in 1968 Tiffany renamed it tanzanite, after the country, and backed it with real marketing muscle. It worked about as well as a rebrand can work: within a few years, an unknown mineral was a recognized fine gemstone, which basically never happens outside of diamonds.
Three colors, one crystal
Tanzanite is strongly trichroic, meaning a single crystal shows three different colors depending on the angle you look through it: blue, violet, and a brownish burgundy. Rough tanzanite straight out of the ground usually looks like a fairly forgettable brown stone. Almost everything on the market today has been heat-treated, around 370-390°C for half an hour, to burn off that brown tone and bring out the blue-violet color. It’s such a standard step that it doesn’t even affect price; buyers just treat it as the last stage of finishing the stone, the same way nobody docks a coffee bean for being roasted.
It’s soft for a gemstone, only 6 to 7 on the Mohs scale, so most jewelers recommend a protective setting if it’s going in a ring. Estimated reserves at Merelani run around 109 million carats, and every bit of it is confined to that one small district. Some of the mining blocks are projected to run dry within the next couple of decades, which is where the long-running rumor comes from that tanzanite could simply run out one day. It picked up an odd bit of official recognition too, added to the December birthstone list in 2002, the first change to that list in over 60 years.
Whatever happens at Merelani, there’s no second location waiting in the wings. This is it.
