A Gem Found Nowhere Else on Earth
Tanzanite is a blue-violet variety of the mineral zoisite, coloured by vanadium. It was discovered in 1967 near the Merelani Hills of northern Tanzania, not far from Mount Kilimanjaro. What makes tanzanite unique among the world's major gemstones is geographic exclusivity: the Merelani Hills are the only known commercial deposit of tanzanite on earth. This is not a relative rarity — it is an absolute one.
Tiffany & Co. recognised the stone's commercial potential immediately after discovery and gave it its trade name. They marketed it as "a gemstone found in two places on earth — in Tanzania and at Tiffany's." That positioning established tanzanite as a luxury stone before most people had heard of it.
Trichroism: Three Colours in One Stone
Tanzanite is strongly trichroic, meaning it displays three different colours when viewed from three different axes: blue, violet, and a brownish-red or burgundy. Cutters orient tanzanite to maximise the blue-violet face-up appearance that defines the commercial standard, but the colour shifts dramatically when tilted, giving the stone a liveliness that is unlike any other blue gem.
Under incandescent light, tanzanite often shifts toward a warmer violet-purple, while daylight brings out the blue. This pleochroism is part of tanzanite's appeal — it is different from any angle, in any light.
Heat Treatment: Universal and Accepted
Unlike sapphire, where heat treatment is a value consideration, tanzanite's heat treatment situation is simple: virtually all tanzanite is heated. Raw tanzanite is typically brownish-red or purple-brown. Gentle heating to approximately 600°C dissolves the brown tones and produces the blue-violet colour the market demands. This treatment is universal, stable, and fully accepted with no value penalty whatsoever.
The D-Block Grading System
The Tanzanite Foundation, established to support the Tanzanian mining industry, developed a grading system for tanzanite using a D-block matrix. Grades range from D1 (most exceptional) to D6 (commercial), assessing colour saturation, tone, and clarity. While not as universally adopted as GIA grading, the D-block system provides a useful framework for comparing stones.
Supply Concerns and the Investment Case
TanzaniteOne, the company that controls most of the Merelani block, has indicated that known reserves could be exhausted within 20–30 years at current extraction rates. Unlike most gems, which have multiple global sources, the end of Merelani means the end of new tanzanite supply. This finite supply narrative is central to the investment case.
However, the tanzanite investment case is more complex than for Kashmir sapphire or Burma ruby. Tanzanite has significant supply currently, meaning short-term price appreciation is not guaranteed. The stone lacks the same prestige cache at major auction houses. For investment purposes, sapphire and ruby have stronger historical track records.
Price Ranges
Commercial tanzanite sells for $100–$300 per carat in smaller sizes. Fine top-colour tanzanite above 5 carats with exceptional blue-violet saturation can reach $600–$800 per carat from quality dealers. Exceptional material above 20 carats in D1 grade has sold at auction for $1,000–$1,200 per carat. These prices make tanzanite accessible compared to other major blue gems, while still offering the "one mine" rarity narrative.
