Bicolour Tanzanite
Bicolour Tanzanite
A window into tanzanite's natural trichroism, preserved in stone
Bicolour tanzanite is a variety of gem-quality zoisite in which two visually distinct colours — most commonly blue or violet alongside brownish-yellow or reddish-brown — appear simultaneously within a single fashioned stone. The phenomenon arises from tanzanite's pronounced trichroism and, in many specimens, from incomplete or deliberately withheld heat treatment. While the overwhelming majority of tanzanite reaching the commercial market has been thermally processed to yield a uniform blue-violet, bicolour examples represent an uncommon and scientifically instructive category that has attracted growing interest among collectors of unheated gems.
Mineralogical Background
Tanzanite is the blue-to-violet gem variety of the calcium aluminium silicate mineral zoisite (Ca2Al3(SiO4)3(OH)), belonging to the orthorhombic crystal system. Its colour derives principally from trace vanadium, with minor contributions from chromium and titanium. Because zoisite is orthorhombic, it is optically biaxial and exhibits three distinct optical axes — and therefore three distinct absorption colours, one along each crystallographic direction. In tanzanite specifically, these pleochroic colours are blue (along the b-axis), violet (along the c-axis), and a reddish-brown to brownish-yellow (along the a-axis). This trichroism is among the strongest observed in any commercially important gemstone.
In the rough, unheated tanzanite crystals frequently display this tricolour character vividly, with different faces of the crystal transmitting markedly different hues depending on the viewing direction. When such a crystal is cut with its table oriented at an oblique angle to the principal optical axes — whether by design or by the constraints of the rough — the finished gem can simultaneously present two of these pleochroic colours to the eye, producing the bicolour effect.
The Role of Heat Treatment
The vast majority of tanzanite sold commercially has been subjected to heat treatment, typically at temperatures between approximately 400 °C and 600 °C. This process oxidises the vanadium chromophore, suppressing the brownish-red component of the trichroism and intensifying the blue-violet saturation that the market prizes. The treatment is stable, undetectable by standard gemmological testing, and so universally applied that the trade considers untreated tanzanite the exception rather than the rule.
Bicolour tanzanite arises in two distinct scenarios. First, a stone may be cut from rough that was never heated, preserving the full natural trichroism; such a gem will show blue or violet alongside the characteristic brown of the a-axis. Second, a stone may have been partially or incompletely heated — whether through uneven thermal penetration of a large crystal or through deliberate restraint — leaving residual brown zones alongside the developed blue-violet. In both cases, the result is a gem that carries visible evidence of tanzanite's underlying optical complexity.
Cutting Considerations
The orientation chosen by the lapidary is decisive in bicolour tanzanite. A cutter working with standard commercial tanzanite orients the table perpendicular to the b-axis or at a compromise angle between the b- and c-axes to maximise the blue-violet face-up appearance and minimise the brown. For bicolour work, the cutter may instead orient the stone so that the table lies at a diagonal to two optical axes, allowing both colours to be visible simultaneously through the crown. Some cutters further exploit the geometry by fashioning elongated shapes — ovals, pears, or cushions — in which one end of the stone appears predominantly blue while the other retains the brown or violet, creating a gradient effect reminiscent of parti-coloured sapphire.
The yield from bicolour-oriented cutting is generally lower than from conventionally oriented material, since the lapidary must sacrifice weight to achieve the desired colour split. This, combined with the relative scarcity of unheated rough, contributes to the premium that fine bicolour specimens can command among specialist collectors.
Identification and Laboratory Reporting
Standard gemmological identification of tanzanite — refractive indices of approximately 1.691–1.700, birefringence of 0.008–0.013, specific gravity near 3.35, and characteristic absorption in the blue-violet region — applies equally to bicolour material. The presence of the brown component is itself a strong indicator of unheated status, since competent heat treatment reliably eliminates it. Laboratories including Gübelin Gem Lab and SSEF have noted that the brown pleochroic colour in tanzanite is associated with the oxidation state of vanadium prior to heating, and its persistence in a fashioned stone is consistent with the absence of effective thermal treatment.
Several major gemmological laboratories will issue reports noting the absence of heat treatment in tanzanite where the evidence supports this conclusion, and bicolour specimens are among the more straightforward cases for such a determination. Lotus Gemology has published commentary on unheated tanzanite identification, noting that the brown component in the pleochroism is a reliable natural indicator. A laboratory report confirming unheated status adds both scientific documentation and market credibility to a bicolour stone.
Provenance
All tanzanite — bicolour or otherwise — originates from a single known source: the Merelani Hills in the Manyara Region of northern Tanzania, near the base of Mount Kilimanjaro. The deposit, divided into mining blocks designated A through D, has been producing gem-quality material since its discovery in 1967. The geological context is a regionally metamorphosed graphitic gneiss sequence, and the vanadium responsible for tanzanite's colour is thought to derive from the surrounding country rock. No other locality has yielded gem tanzanite in commercial quantities, making geographic origin determination straightforward by default.
Market Position and Collector Interest
Bicolour tanzanite occupies a niche but genuine position in the collector market. The dominant commercial preference remains for strongly saturated, uniformly blue-violet heated tanzanite, and this preference governs pricing at the mass-market level. Bicolour material does not typically command higher prices in the general trade; indeed, stones with visible brown are often discounted relative to well-heated equivalents of comparable size.
Among collectors of unheated gems and those drawn to optical phenomena, however, bicolour tanzanite is valued precisely for what the brown component represents: an unaltered record of the mineral's natural state. The growing collector interest in unheated coloured stones — paralleling the long-established premium for unheated sapphire and ruby — has begun to extend to tanzanite, and fine unheated bicolour examples with strong colour contrast and good transparency are increasingly sought at specialist auction and through dealers who focus on natural, untreated material.
Sizes above five carats in well-cut bicolour material with clearly defined colour zones are uncommon. Very large examples — above fifteen carats — are rare enough to be noteworthy. As with all tanzanite, the finite nature of the Merelani deposit lends a long-term scarcity argument to the category, though this consideration applies broadly to tanzanite as a species rather than specifically to bicolour material.
Care and Handling
Tanzanite has a Mohs hardness of 6 to 6.5 and perfect cleavage in one direction, making it moderately susceptible to abrasion and vulnerable to sharp blows. These characteristics apply equally to bicolour specimens. Ultrasonic and steam cleaning are inadvisable; warm soapy water and a soft brush are the recommended cleaning method. Because bicolour tanzanite is often unheated, owners and dealers should exercise particular care to avoid inadvertent heat exposure — from jeweller's torches during setting work, for instance — which could permanently alter the stone's colour balance and eliminate the brown component that defines its character.