About Tanzanites
Tanzanite is the trade name for the blue-to-violet variety of the mineral zoisite, found commercially in a single hill-belt at Merelani, near the foothills of Mount Kilimanjaro in northern Tanzania. The deposit was discovered in 1967 by a Maasai tribesman and a Goan-Indian prospector, and the stone was introduced to the international gem market by Tiffany & Co. the following year under the now-famous name “tanzanite,” chosen by Tiffany's chairman to reflect both the stone's country of origin and to suggest the rarity and exclusivity of the supply. The Merelani deposit is unique in world gemmology in that no other commercial source of gem-quality blue zoisite has ever been identified; tanzanite is, by any reasonable measure, one of the rarest commercial gemstones on the international market.
Chemically, tanzanite is a calcium aluminium silicate (Ca2Al3(SiO4)3(OH)) coloured by trace vanadium. The most distinctive optical feature is exceptionally strong trichroism — the stone displays three distinct colours from different crystallographic directions: a deep blue along one axis, a violet along the second, and a burgundy-red along the third. The lapidary's task in cutting tanzanite is to orient the finished stone to maximise display of the blue and violet directions and to minimise the burgundy — a difficult choice that materially affects both the apparent colour and the recoverable weight from each piece of rough.
Heat treatment is universal in commercial tanzanite. Most Merelani rough emerges from the ground in a brownish-yellow colour with only a hint of blue; gentle heating to approximately 600°C in air converts the trivalent vanadium to a form that produces the saturated blue and violet for which the stone is famed. This treatment is regarded across the trade as standard and is fully accepted in the market, but every AGL tanzanite certificate states the heat-treatment status explicitly: “Indications of heating.” Naturally blue tanzanite — rough that emerges from the deposit already saturated — does exist but is genuinely rare and is documented separately as “No indications of heating” on the AGL report. Such stones command significant premiums in the connoisseur market.
Tanzanite is graded in the trade on a quality scale running from D (entry-level) through A, AA, AAA, and AAAA (premium), with the AAA and AAAA grades reserved for the deeply saturated, slightly violetish-blue material that most closely resembles a fine Burma sapphire. The principal limitations of tanzanite as a fine jewellery stone are its relatively modest hardness (6.5 to 7 on the Mohs scale, considerably softer than corundum or beryl) and its perfect cleavage along one axis, which makes the stone vulnerable to impact damage when set in rings. Tanzanite is best suited to earrings, pendants, and brooches where it can be appreciated for its remarkable colour without exposure to the impacts to which a ring stone is regularly subjected. The AGL tanzanite certificate confirms the natural mineral species (not synthetic forsterite, not coated quartz), declares the heat-treatment status, and provides a full colour and clarity grade on the established trade scale.
How AGL certifies Tanzanites
Every tanzanites submitted to an AGL laboratory is examined by at least two senior gemologists and analysed using our standard instrumental package: refractive-index and specific-gravity measurement, microscopic examination under dark-field and fibre-optic illumination, UV-VIS-NIR absorption spectroscopy, FTIR spectroscopy for treatment detection, and LIBS or ED-XRF for trace-element analysis where origin is in question. Every conclusion on the certificate — species, variety, treatment, origin — is supported by recorded analytical data, and every certificate is recorded in our secure verification database for online authentication at any time.
For full service descriptions and turnaround times, see our Services page, or contact your nearest AGL laboratory for a written quotation.
