Tanzanite

Tanzanite is the blue-to-violet gem variety of the mineral zoisite, a calcium aluminum silicate. Its striking color is caused by trace vanadium, and most rough is heat-treated to remove brownish tones and bring out pure blue and purple hues.

Discovered only in 1967 and named by Tiffany & Co. after its sole source, the gem occurs in a small area near Mount Kilimanjaro in Tanzania. Its single-source rarity is a major part of its appeal.

Tanzanite is strongly trichroic, showing blue, violet, and burgundy depending on the viewing angle, which cutters orient to maximize the desirable blue-violet face-up color.

Tanzanite formed roughly 585 million years ago during the metamorphism associated with the East African continental collision. It occurs in vanadium-bearing metamorphic rocks, crystallizing in pockets and veins within graphite-rich gneisses and schists.

The only commercial deposit lies in the Merelani Hills near Arusha, Tanzania, in a geologically constrained zone. This extreme geographic restriction, combined with mining challenges, makes the supply finite and contributes to the gem's collectible status and price volatility.

Tanzanite's most diagnostic feature is its strong trichroism: rocking a faceted stone reveals blue, violet, and reddish-brown flashes from different directions. It has a vitreous luster, hardness of 6.5-7, and is doubly refractive with a white streak.

It is softer and less durable than sapphire and has distinct cleavage, making it sensitive to knocks. Look-alikes include sapphire (much harder at 9, weaker pleochroism), iolite (different pleochroic colors, lower SG), and blue spinel or glass. The combination of moderate hardness, strong trichroism, and Tanzanian provenance is characteristic.

Tanzanite is used almost exclusively as a faceted gemstone in rings, pendants, and earrings, prized for its saturated blue-violet color. It was added as a December birthstone in 2002.

Because it has only one source and moderate hardness, it is best suited to earrings and pendants or protected ring settings rather than rough daily wear. Value rises sharply with size and depth of blue-violet color; intense "D-block" stones command premiums. Metaphysically, tanzanite is associated with insight, transformation, and communication. Careful handling is advised due to its cleavage and sensitivity to sudden temperature change.