Tanzanite Buying Guide
Tanzanite is the name given to the rich blue-violet variety of the epidote-group mineral, zoisite. The gem was first discovered in Tanzania in 1967 and was named after its country of its origin, Tanzania, by the famous New York jeweler, Louis Comfort Tiffany.
Tanzanite has what many would consider the finest blue hue in the world of gemstones, a color which often leaves even the finest sapphires lacking. The gem is strongly trichroic.
A chromium-colored green tanzanite is also found in Tanzania.
A stunning 45-ct. tanzanite from Pala International. Photo: Wimon Manorotkul; Gem: Pala International
Tanzanite is extremely light sensitive, with incandescent lighting tending to shift its color to the violet side. The best pieces show an intense blue under any light.
In terms of clarity, tanzanite does occur in reasonably clean crystals. Thus when buying, the standard is eye-clean stones.
In the market, tanzanites are found in a variety of shapes and cutting styles. Ovals and cushions are the most common, but rounds are also seen, as are other shapes, including emerald cuts, trillions, etc. Cabochon-cut tanzanites are not often seen.
Tanzanite prices probably fluctuate more than any other gem, largely due irregular production at the mines. For extremely fine stones of less than 50 cts., prices sometimes top $1000/ct. at retail.
Tanzanites sometimes occur in extremely large sizes, with faceted gems of hundreds of carats in existence. The most popular sizes for jewelry use are those below 20 cts.
The only locality for tanzanite is Merelani in Tanzania.
Virtually all gem tanzanite of a blue color has been heat-treated to enhance the color.
Tanzanite has never been synthesized, but a number of imitations exist. The most common is blue-violet glass. A synthetic fosterite has also been used as an imitation.
This suite of tanzanites shows the typical range of colors possible. The ideal would be the center stone, which is not too light or dark. Photo: Wimon Manorotkul; Gems: Pala International
Properties of Tanzanite
|Hardness (Mohs)||6 to 7|
|Specific Gravity||3.35 (+ 0.10, -0.25)|
|Refractive Index||1.691–1.700 (0.008–0.013); Doubly refractive, biaxial positive|
|Colors||Blue, violet, green, brown|
|Cleavage||Perfect in one direction|
|Phenomena||Cat’s eyes are known. Many tanzanites display a color shift from blue in daylight to violet in incandescent light.|
|Handling||Ultrasonic: Unsafe; never clean tanzanite with ultrasonic cleaners
Steamer: Not safe
The best way to care for tanzanite is to clean it with warm, soapy water.
Acids: Attacked by hydrochloric and hydrofluoric acids
Heat: Fuses under jeweler’s torch; sudden temperature changes may cause cracking.
|Enhancements||Virtually all tanzanite is heat treated (approx. 500°C), which converts the brown pleochroic color to blue.|