Tanzanite is blue-violet zoisite, discovered in 1967 in northern Tanzania near Mount Kilimanjaro, the only known commercial deposit of gem-quality zoisite with this colour. Blue sapphire is corundum, found in multiple countries across multiple continents. In fine examples of each, the colours can be strikingly similar face-up in certain lighting, which makes the comparison commercially relevant even though gemmologically the stones are easily distinguished.
Optical identification: immediate and definitive
The refractometer separates tanzanite and sapphire immediately. Tanzanite reads 1.690ÔÇô1.702 with birefringence of 0.009. Sapphire reads 1.762ÔÇô1.770 with birefringence of 0.008. The different RI ranges are non-overlapping and definitive. Tanzanite also shows exceptionally strong trichroism, three distinct pleochroic colours (blue, violet, red) visible in the dichroscope, while sapphire shows weaker dichroism. A Chelsea filter gives different readings. Spectroscopy is definitive if needed. Any competent gemmologist identifies the two stones in under a minute.
Durability: the critical practical difference
Tanzanite scores 6.5 on the Mohs scale, compared to sapphire’s 9. Tanzanite also has perfect cleavage in one direction, meaning it can fracture with a sharp impact. This combination makes tanzanite inappropriate for daily-wear rings without protective settings and significantly limits its durability compared to sapphire. A tanzanite ring worn daily will sustain surface abrasion over years of use; a sapphire ring in the same condition will not. This is not a reason to avoid tanzanite. It is a reason to choose appropriate settings and appropriate occasions. Pendants and earrings are better suited to tanzanite than unprotected rings.
Colour: the buyer’s comparison
The finest tanzanite shows a vivid blue-violet with excellent saturation that shifts between blue and violet depending on viewing angle and light source. The finest blue sapphire from Kashmir or Burma shows a pure blue with minimal secondary hues. Whether one is more beautiful than the other is subjective; they are genuinely different colours despite occasional visual similarity. Collectors who prioritise pure blue will always prefer sapphire. Collectors who appreciate the violet component and the pleochroic shift find tanzanite interesting in a way sapphire is not.
Price and value
Fine tanzanite of five carats with vivid blue-violet sells at $400ÔÇô1,500 per carat. Fine blue sapphire at comparable size and quality sells at $2,000ÔÇô15,000 per carat depending on origin. Tanzanite’s single-source supply, one deposit in Tanzania, is sometimes cited as a scarcity argument. In practice, tanzanite prices have not appreciated significantly in the collector market over the past decade. The stone has a strong commercial market but has not built the collector premium that Kashmir sapphire or fine Burma ruby has established. For investment purposes, sapphire has a stronger track record; for enjoyment at accessible prices, tanzanite offers excellent value.