silicates
Tanzanite
- Crystal system
- orthorhombic
- Hardness
- 6–7 on the Mohs scale
- Lustre
- vitreous
- Colour
- Violet-blue, strongly pleochroic
- Where it forms
- In metamorphic rocks at Merelani in Tanzania, under conditions that are apparently unique — no comparable deposit has been found anywhere else in fifty years of looking.
- Chiefly from
- Merelani Hills, Tanzania
Why it is that colour
Vanadium. Rough material is usually brownish, and heating to around 600 degrees converts the vanadium to the state that gives blue. Nearly all tanzanite on the market has been heated, and this is standard and generally disclosed.
Safety
Safe to handle, but fragile: perfect cleavage in one direction and sensitivity to sudden temperature change mean it chips and cracks more easily than its hardness suggests. Not a good ring stone.
Where it comes from
A single deposit of a few square kilometres, divided into blocks worked by a large operator and by thousands of artisanal miners. Conditions in the artisanal workings have been poor, with deaths from flooding and collapse; a wall was built around the mining area in 2018 to control smuggling, which has itself been controversial.
Historical use
Adopted as a birthstone for December in 2002, which is a good measure of how recent the whole apparatus around it is.
Recorded as history and attributed where possible. That a stone was used for something establishes that people used it, not that it worked.
What is attributed to it, and since when
Found in 1967 and named tanzanite by Tiffany in 1968, after the country, in preference to the mineralogically correct blue zoisite — which Tiffany reportedly thought sounded too much like suicide. It has no history before 1967 and every meaning attached to it has been assembled since.
Described, not endorsed. Dating a claim is not the same as assessing it — but an association that first appears in a book published in 1985 is not an ancient inheritance, whatever the label says. A sincere account of feeling better is compatible with the stone doing nothing — see the placebo effect.
Sold as this, but not
Worth knowing
Tanzanite is genuinely rare in a way most "rare" gems are not. It comes from one deposit, a few square kilometres in northern Tanzania, and geologists have not found another. Estimates of how long it will last vary, but it is a finite and identifiable quantity, unlike almost anything else in this collection.
Everything else about it is recent and constructed. It was found in 1967, named by Tiffany's marketing department in 1968, and made a birthstone in 2002 — birthstone lists being commercial documents maintained by jewellers' associations, revised when it suits them. The 1912 American list, which most modern lists descend from, was drawn up by the National Association of Jewelers.
A genuinely scarce stone with an entirely manufactured meaning is a useful thing to hold in mind. The scarcity is real. The significance was written by an advertising department, and this is one of the few cases where we can name the year.
Identity checked against IMA-CNMNC master list, June 2025.