Tanzanite shifts from violet to blue depending on the light. Here's why it does that, where on earth it comes from, and why both facts matter to buyers.
Hold a tanzanite under an incandescent bulb. Distinctly violet. Take it outside into daylight. Suddenly blue. Point a UV torch at it: almost burgundy. Same stone, three different personalities. This optical behaviour isn't a quirk or a treatment effect — it's physics, built into the crystal.
WHY TANZANITE CHANGES COLOUR
Tanzanite is strongly trichroic. That means it absorbs and transmits light differently along its three optical axes. Rotate the stone, change the light source, or tilt your viewing angle — and the colour shifts.
The three "faces" of tanzanite:
- Along one axis: blue
- Along a second axis: violet or purple
- Along the third: burgundy or reddish-brown (rarely visible in finished stones)
The cutter decides which colours dominate. A stone oriented to show more blue face-up is generally considered more valuable. A stone that tilts toward violet is still beautiful — just differently priced. This is why two tanzanites of identical carat weight and saturation can look dramatically different: the orientation of the cut determines everything.
Trichroism is caused by the stone's vanadium content interacting differently with light depending on polarisation. It's the same reason Alexandrite changes colour (though the mechanism differs). In tanzanite, the shift is usually from violet to blue rather than green to red — but the drama is comparable.
WHERE TANZANITE ACTUALLY COMES FROM
Here's the part most buyers don't know: tanzanite doesn't just come from Tanzania. It comes from one specific location within Tanzania — a 4km × 2km mining block near the town of Merelani, at the base of Mount Kilimanjaro.
That's the entire world supply.
Geologists have searched extensively. No other economically viable deposit has ever been found. The conditions that produced tanzanite — a precise convergence of vanadium-bearing metamorphic rocks, extreme heat and pressure, and specific fluid chemistry around 585 million years ago — appear to have existed only in this narrow zone.
The Tanzanian government administers the mining zone as four Blocks (A, B, C, D). Block C, operated by TanzaniteOne, is the dominant producer. The government has periodically restricted export of rough material to encourage local cutting and value-adding within Tanzania.
At current extraction rates, geologists estimate the Merelani deposit has 20–30 years of commercially viable mining life remaining.
WHY BOTH FACTS MATTER TO YOU AS A BUYER
The trichroism: always view tanzanite in multiple light sources before buying. A stone that looks spectacular in the jeweller's warm incandescent lighting should also be evaluated in natural daylight. Some stones lose their magic outdoors. The best tanzanites look stunning in both.
The single source: there is no "generic tanzanite." Every tanzanite that has ever been cut came from the same patch of ground in northern Tanzania. When that ground is depleted, the supply ends. This is not speculation — it is geology. It is the primary reason that investment-grade tanzanite (especially stones above 5ct with vivid, predominantly blue colour) has appreciated consistently over the past 20 years.
BUYING TIP
For maximum colour shift visibility, look for well-cut stones with strong saturation in the 1–5ct range. Under-saturated stones still show trichroism but the shift is less dramatic. The most sought-after tanzanites show a vivid blue-violet face-up with flashes of violet visible from the side — a combination that's rare and increasingly expensive to source.