One Hill on Earth. Nowhere Else.

Every other stone in the Carat Brief Index comes from several places. Sapphire from Kashmir, Ceylon, Madagascar. Ruby from Mogok and Montepuez. Emerald from Colombia and Zambia. Spinel from Burma and Tanzania. Alexandrite from the Urals and Brazil. Origin is a variable, and this newsletter has spent weeks on the premium it commands.

Tanzanite has no variable. It comes from one hill. A single patch of Tanzanian ground, roughly four kilometers by two, at the foot of Mount Kilimanjaro, is the only place on earth this gem has ever been found. It was discovered barely sixty years ago, by a grassfire, and named by Tiffany. It sits at the very bottom of the index at $1,200 per carat. And it is the purest scarcity story on the whole table, for one simple reason: when the hill is empty, there is no second hill.

→ A quiet August saleroom. No realized colored-stone print landed this week. The calendar sits between the June New York sales and the autumn Geneva season, the moment to study the one stone whose entire supply comes from a single deposit.

→ The single-source fact. Tanzanite is mined only in the Merelani Hills near Arusha, in a deposit of about four by two kilometers, divided into four blocks. There is no other commercial source anywhere. The trade and the mineralogists broadly agree the deposit is finite and could be materially depleted within a few decades (Smithsonian; Gem Society; geology references).

→ A stone that is treated, and that is fine. In contrast to the spinel and alexandrite of recent weeks, nearly all tanzanite is heat-treated to turn brownish rough into blue-violet. The treatment is stable, universal and disclosed by default. Nature, in fact, did it first: the 1967 grassfire that revealed the gem is exactly the process a jeweler now repeats in a kiln (Gem Society).

The index tracks the 8 reference stones every week. Each row holds until a new dated transaction moves it. A quiet week leaves all eight unchanged. This week's focus is the tanzanite row, the lowest line on the table.

Auction prices realized, not retail. Sources: Christie's, Sotheby's, Gemfields, Bonhams, GemGuide, Pala International.

Notable this week: tanzanite is the cheapest line in the index, and that is the point. Everything above it is priced partly on rarity of origin. Tanzanite is priced almost entirely on it, because origin is all it has. No Mogok versus Mahenge debate, no Kashmir premium to chase. One deposit, and a clock on it. It is the single cleanest test of whether scarcity alone can carry a gem.

One deposit, and only one. Nearly every important gem is found in several countries, which is what lets a lab charge for an origin report and a dealer charge for a Kashmir or a Colombian. Tanzanite breaks that model. The world's entire supply comes from the Merelani Hills at the base of Kilimanjaro, roughly eight square miles worked in four blocks, A through D. Outside that patch, gem tanzanite does not exist. It is the only major gemstone on earth with a genuinely single source.

Born in a fire, in living memory. In 1967, a grassfire swept the Merelani Hills, and when it passed, a Maasai herder noticed blue-violet crystals on the scorched ground. The fire's heat had turned dull brown zoisite vivid blue, the same treatment a jeweler applies today. A prospector, Manuel de Souza, took them for sapphire; tests proved otherwise, a blue zoisite never before seen in gem quality. Almost every other gem in this index was known to antiquity. Tanzanite was found within a living lifetime.

Tiffany made the name, and the market. In 1968, Tiffany and Company began marketing the new gem. They could have sold it as blue zoisite. They chose tanzanite, for the country and for the selling power of a single-origin story, and called it the most beautiful blue stone discovered in two thousand years. They made it a December birthstone and built a demand that had not existed two years earlier. It is the rare case where the marketing is part of the gemology: tanzanite's value is inseparable from the campaign that named it.

The color is an orientation, not a fact. Tanzanite is strongly pleochroic: the same crystal shows blue, violet, or burgundy from different directions, and the cutter chooses which to face up. The finest stones are deep sapphire blue with violet flashes, and the color intensifies with size, a five carat top-color stone far more saturated than a one carat of the same grade. Here size does not just add weight. It adds color.

Trade ranges. Unlike the rest of the index, the axis is size, not origin, because there is only one origin. Color saturation rises with size, which is the basis for the premium (trade and lab-guide references).

The honest verdict. Tanzanite is the index's clearest scarcity bet and its weakest blue-chip. The scarcity is real and unusually legible: one mine, finite, in living memory. But three things keep it cheap. It is abundant at the low end, so only fine large stones are genuinely rare. It is heat-treated, so there is no untreated premium to capture. And its prestige was manufactured by a single marketer rather than earned over centuries, which makes its demand more fashion-dependent than sapphire's. This is not a stone that will quietly compound like Kashmir. It is a speculation on one specific idea: that a single, finite source, marketed well, reprices when the ground starts to empty.

1. This is a pure supply bet, and nothing else. Strip away treatment, phenomenon and origin-hunting, because tanzanite offers none of them as a lever. What is left is geography: one deposit, finite, with a depletion clock the whole trade acknowledges. If you believe that clock, the case is simply that the ground runs out. If you do not, you are buying an affordable blue. Be honest about which bet you are making.

2. Treated is the norm, so price the color, not the paper. Assume any blue-violet tanzanite is heat-treated. The treatment is stable and standard, and it is not a flaw. It does mean there is no untreated premium here, unlike spinel or alexandrite. The premium is entirely for saturation and size, so grade the color and buy weight where the color deepens.

3. The value lives above five carats, and there you certify. Because saturation climbs with size, the meaningful money is in fine, large, top-color stones, not melee. That is also where grade spreads widen enough to justify a lab report, and where simulants and synthetic zoisite are worth ruling out. Below a carat, tanzanite is a pretty stone. Above five, in top color, it is the actual asset.

→ Merelani output and formalization. Tanzania walled the deposit in 2018 and pushed exports through licensed channels. Those moves affect the timing of supply more than the total. Watch official rough flows for any signal that production is tightening structurally rather than seasonally.

→ The depletion narrative. Any credible new estimate of how much gem-grade rough remains in the four blocks would move sentiment immediately, because scarcity is the entire thesis. This is the number to watch for tanzanite, the way grade-per-tonne is the number for ruby.

→ Marketing support. Tanzanite's demand was built by a campaign and still leans on one. Watch whether the trade bodies and major houses keep investing in it, because for this stone, unusually, marketing is a supply-and-demand input.

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If tanzanite tempts you as an investment rather than a stone you simply like, write down one sentence before you buy: what has to be true for this to appreciate. For tanzanite the answer is unusually clear, and unusually singular. The Merelani deposit has to empty faster than demand fades. That is the entire trade. If you believe it, buy fine and large. If you are not sure, enjoy the color and expect nothing more, because everything else about this gem is already priced.

— Carat Brief

Carat Brief is editorial. Nothing here is investment, legal, or tax advice. Gemstones are illiquid and require certified verification before purchase. Work with credentialed labs: GIA, AGL, SSEF, Gübelin, Lotus Gemology.

Sources: Smithsonian National Museum of Natural History; Gem Society; geology.com; trade and lab-guide references. Per-carat ranges are trade benchmarks and vary by color saturation, size and clarity.