The mineralogy: zoisite and the tanzanite variety
Tanzanite is a variety of zoisite, a calcium aluminium silicate mineral of the epidote group with the chemical formula Ca₂Al₃(SiO₄)₃(OH). Zoisite is orthorhombic (it crystallises in the orthorhombic system, with three mutually perpendicular axes of different lengths), which is directly relevant to understanding trichroism: the orthorhombic structure means the crystal interacts differently with light along each of its three principal axes, producing three different apparent colours depending on viewing direction (GIA Gem Reference Guide, 2006, pp. 96–97; Klein, C., Manual of Mineral Science, 2002).
Zoisite occurs in many colours: grey, green, pink (thulite), yellow, and the blue-violet variety that is tanzanite. The tanzanite variety requires vanadium as the primary colourant. Only the Merelani Hills have produced tanzanite in gem quality and commercial quantity, despite the occurrence of zoisite in other geological settings worldwide (GIA; Wise, 2016).
Physical properties: Mohs hardness 6–7 (softer than corundum and beryl, an important care consideration); perfect cleavage in one direction (making tanzanite more prone to breakage from sharp impact than its hardness alone suggests); specific gravity approximately 3.35; refractive index approximately 1.69–1.70. The perfect cleavage along the {010} plane means tanzanite should be set in protective settings and handled with more care than its hardness number suggests (GIA Gem Reference Guide, 2006; Wise, 2016).
Colour and the vanadium mechanism
The blue to blue-violet colour of heat-treated tanzanite is produced primarily by vanadium (V³⁺) in octahedral coordination within the zoisite crystal structure. Chromium (Cr³⁺) contributes secondary colour in some stones, producing a slightly stronger blue or violet depending on its concentration relative to vanadium. The specific combination of vanadium and the orthorhombic crystal field produces tanzanite's distinctive colour: a rich blue with a noticeable violet secondary, sometimes described as "velvety" in analogy to Kashmir sapphire, though the optical mechanism is different (Nassau, K., American Mineralogist, 63:219–229, 1978; GIA Gems and Gemology research on tanzanite colour).
The violet component in tanzanite is one of its defining characteristics and distinguishes it clearly from blue sapphire in side-by-side comparison. Fine tanzanite shows a blue that is simultaneously deeper and more violet than sapphire's pure blue, with a quality sometimes compared to the colour of twilight. The violet component is more prominent in incandescent light, less so in daylight, a mild colour shift (not a dramatic change like alexandrite, but a perceptible one that affects how the stone appears across different lighting environments) (GIA; Wise, 2016, pp. 153–158).
Tanzanite trichroism: three different colours visible along the three principal crystallographic axes. The a-axis shows vivid blue; the b-axis shows violet; the c-axis shows burgundy or reddish-brown. Heat treatment eliminates or reduces the c-axis colour, leaving the commercially valued blue-violet. The cutter orients the table to display the a-axis blue face-up. Source: GIA Gem Reference Guide (2006).
Trichroism: why raw tanzanite is brown
Trichroism is the property of a crystal showing three different colours when viewed along three different crystallographic axes. In orthorhombic minerals like zoisite, the three principal axes interact with light differently enough to produce visibly distinct colours. Tanzanite's trichroism is among the most pronounced of any gemstone: the three colours (blue, violet, and burgundy-to-brown) are clearly distinct rather than subtly different (GIA Gem Reference Guide, 2006, pp. 96–97).
Raw tanzanite from the Merelani Hills is typically brown to reddish-brown in appearance because the c-axis colour (burgundy/brown) dominates in randomly oriented rough crystals, and the brown absorbs the blue and violet wavelengths that the a-axis and b-axis transmit. The stone in its natural state, before heat treatment, is commercially unattractive. The blue and violet colours that make tanzanite commercially valuable are present in the raw crystal, they are visible when the crystal is oriented correctly and viewed along the appropriate axis, but they are overpowered by the brown in most orientations.
Heat treatment at approximately 500–600°C in an oxidising atmosphere changes the oxidation state of the iron (Fe²⁺ to Fe³⁺) that contributes to the brown component, effectively eliminating or greatly reducing the c-axis brown colour. After heating, the trichroism remains but the three apparent colours become blue, violet, and a reduced burgundy, and the stone oriented with the blue axis facing up shows the vivid blue-violet that the market recognises (GIA; Wise, 2016; Nassau, K., Gems Made by Man, 1980).
Heat treatment: universally applied and universally accepted
Virtually all commercial tanzanite has been heat treated. This is not a source of commercial concern in the tanzanite market, unlike the heat treatment of ruby and sapphire where unheated status commands a significant premium. Tanzanite's heat treatment is different in nature from corundum heat treatment:
It is almost universal: Estimates suggest over 99% of commercial tanzanite has been heat treated. Untreated tanzanite of commercial quality (blue-violet without heating) exists but is extremely rare, it requires the crystal to have been oriented so the blue-violet axis dominates without any brown component, which is an unusual geological occurrence in the Merelani Hills material (GIA; Tanzanite Foundation).
It is reversible in principle: Unlike heat treatment of corundum which permanently reorganises trace element distribution, tanzanite's heat treatment changes iron oxidation state, which is in principle reversible under reducing conditions. In practice, commercial tanzanite retains its colour indefinitely under normal conditions (GIA; Wise, 2016).
It is accepted without premium penalty: No premium exists for unheated tanzanite equivalent to the unheated premium for ruby or sapphire. The market treats all commercial tanzanite as heat treated by default. A seller who claims "natural colour, unheated" tanzanite is making a claim that requires laboratory verification, and even verified unheated tanzanite does not command the dramatic per-carat premium that unheated corundum does (GIA; AGL; Tanzanite Foundation).
It is disclosed in trade: GIA and other major laboratories note heat treatment on tanzanite certificates. The notation does not reduce the stone's commercial value. AGTA and CIBJO both classify tanzanite heat treatment as accepted and routinely disclosed without commercial penalty (AGTA treatment codes; CIBJO Coloured Stone Blue Book).
The single source: Merelani Hills and what it means
Tanzania has formally declared the Merelani Hills deposit the world's only known source of tanzanite, and this has been the geological consensus since intensive prospecting following the 1967 discovery found no other occurrences of comparable gem-quality tanzanite elsewhere on earth. This single-source status creates commercial dynamics not seen in any other major gem species:
The supply of tanzanite is finite, predictable in its endpoint, and concentrated in a single political and geological jurisdiction. When the Merelani deposit is exhausted, tanzanite production ends globally. No new deposits can be developed, because no new deposits are known to exist. The Tanzanian government has periodically stated concern about depletion rates relative to reserve estimates, and formal reserve studies have suggested that at current production rates, the primary deposit may be substantially depleted within one to three decades (Tanzanite Foundation; Tanzanian Ministry of Minerals; various geological assessments cited in GIA Gems and Gemology).
Tanzanite's single-source constraint compared with other major coloured gems. Ruby, sapphire, emerald, and alexandrite all have multiple producing countries. Tanzanite has exactly one: the Merelani Hills of Tanzania. When this deposit is exhausted, tanzanite production ends globally. Source: GIA; Tanzanite Foundation; Geological Survey of Tanzania.
Tanzanite quality: the key factors
Tanzanite quality is evaluated on the same colour-first framework as other coloured stones, with quality factors specific to the species:
Colour: The optimal tanzanite colour is a vivid to strong blue with a clearly visible violet secondary hue. The trade describes this as "ultramarine" or "royal blue with violet." The finest tanzanite shows a deep, saturated blue-violet that is richer than sapphire's pure blue, the violet component is part of the value rather than a modifier. Pale, washed-out tanzanite (too light in tone) and overly dark tanzanite (approaching inky black at large sizes) both fall below the optimal range (GIA; Wise, 2016, pp. 158–162).
Clarity: GIA classifies tanzanite as Type I for clarity: most fine tanzanite is eye-clean. Unlike emerald (Type III) where inclusions are expected, or ruby (Type II) where significant inclusions are common, high-quality tanzanite is typically clean. An included tanzanite at fine colour and significant size should carry a meaningful discount from eye-clean equivalents (GIA Colored Stone grading).
Size premium: The size premium in fine tanzanite is pronounced. Fine tanzanite above 5 carats with vivid colour and eye-clean clarity commands significant per-carat premiums over smaller equivalents. The Tanzanite Foundation uses a grading system (D Block for the finest quality) that effectively maps the quality-size relationship (Tanzanite Foundation; Wise, 2016).
Synthetic tanzanite and simulants
No commercially produced synthetic tanzanite exists at the time of writing. Tanzanite synthesis has been explored but the technical challenges of replicating the specific zoisite crystal structure at commercial scale have not been overcome in a way that produces commercially significant quantities. This distinguishes tanzanite from ruby, sapphire, emerald, and alexandrite, all of which have significant synthetic production (GIA; Nassau, 1980).
Tanzanite simulants exist: blue synthetic corundum (sapphire-coloured), blue synthetic spinel, and blue glass are all used as tanzanite simulants at the lowest price points. Iolite (cordierite), a naturally pleochroic blue-violet mineral, is occasionally misrepresented as tanzanite. A refractometer immediately distinguishes tanzanite (RI approximately 1.69–1.70) from synthetic corundum (RI approximately 1.762–1.770) and from iolite (RI approximately 1.522–1.578). A trained gemologist distinguishes all simulants in minutes (GIA Gem Reference Guide, 2006).
Frequently asked questions
Is tanzanite rarer than diamond?
Tanzanite is frequently marketed with the claim that it is "1,000 times rarer than diamond" or similar. This figure comes from Tiffany and Company marketing language from the late 20th century and refers to the geographical exclusivity of the source: tanzanite comes from one deposit in one country, whereas diamonds come from many countries. The comparison is not a scientifically rigorous measure of relative abundance, diamond production by weight is also finite, and the economics of rarity are complex. What is accurate and important: tanzanite comes from a single deposit with finite reserves, no other source exists, and when the Merelani deposit is exhausted, tanzanite is gone. In that geological and supply sense, its rarity is more absolute than that of any other major commercial gemstone. The "1,000 times" figure is marketing language; the underlying supply constraint is real (Tanzanite Foundation; GIA).
Why is virtually all tanzanite heat treated if the blue colour exists naturally?
The blue-violet colour exists in the raw crystal along specific axes, but the trichroism of raw tanzanite material means that in most orientations, the brown c-axis colour dominates the appearance. A raw tanzanite crystal held at random shows brownish rather than blue-violet in most directions. Heat treatment eliminates the iron-related brown component and allows the vanadium blue-violet to express fully across all orientations. The resulting colour is the same vanadium blue-violet that existed in the crystal before treatment, the heat treatment removes an interfering colourant rather than adding a new one, which is why the resulting colour is considered the stone's natural expression and why the treatment is accepted without commercial penalty (GIA; Nassau, 1980; Wise, 2016).
Does tanzanite make a good everyday ring stone?
Tanzanite is softer (Mohs 6–7) and has perfect cleavage, making it less suitable for daily wear rings than corundum (Mohs 9, no cleavage) or even emerald (Mohs 7.5–8, indistinct cleavage). A tanzanite ring worn daily will scratch faster than sapphire or ruby, and a sharp impact can cause cleavage fracture. Tanzanite is better suited for earrings, pendants, and occasional-wear rings where impact risk is lower. If worn as a ring, a protective bezel or half-bezel setting is recommended over a high-profile prong setting. This is a physical property of the mineral, not a quality deficiency, tanzanite is simply less hard than corundum (GIA; Wise, 2016).
Is there a Jyotish association for tanzanite?
Tanzanite is not one of the nine Navratna stones as traditionally defined, because the Navratna tradition predates the discovery of tanzanite by millennia. Some contemporary practitioners include tanzanite as an alternative stone for Saturn (Shani) based on its blue-violet colour, treating it as a substitute for blue sapphire (Neelam) when Neelam is not available or affordable. This is not a traditional Jyotish prescription; it is a modern extension by individual practitioners. Whether tanzanite is acceptable for this purpose is a matter for the specific practitioner. From a gemological standpoint: tanzanite is a natural stone with a vivid blue-violet colour and natural colour enhancement through heat treatment, which most practitioners would consider equivalent in status to the accepted heat treatment of commercial sapphire.
Sources cited in this article
- GIA Gem Reference Guide. (2006). Gemological Institute of America. (pp. 96–99)
- Nassau, K. (1978). "The Origins of Color in Minerals." American Mineralogist, 63:219–229.
- Nassau, K. (1980). Gems Made by Man. Chilton Book Company.
- Wise, R.W. (2016). Secrets of the Gem Trade (2nd ed.). Brunswick House Press. (pp. 153–162)
- Klein, C. (2002). Manual of Mineral Science (22nd ed.). John Wiley and Sons. (Zoisite entry)
- Tanzanite Foundation. Supply information and reserve estimates. tanzanitefoundation.com.
- AGTA. Treatment disclosure codes (tanzanite heat treatment). agta.org.
- CIBJO. Coloured Stone Blue Book, current edition. cibjo.org.
- GIA Colored Stone grading and identification. gia.edu.