Tanzanite Jewelry and Gemstone Information

7 carat Blue-Violet Tanzanite, re-polished by Daniel Stair

Truly a modern gemstone, transparent zoisite of a naturally reddish brown color which can be heated to a stable blue to violet, was discovered in the shadow of Mt. Kilamanjaro in 1969. Although other varieties of opaque zoisite were well known, they made no impact on the gem market. Tanzanite's rise to prominence among retail jewelers and the general public has been rapid and dramatic. Naturally trichroic, the species shows different colors when viewed through each of its three crystal axes: blue, red-violet and yellow-green.

DataValue
NameTanzanite
Is a Variety ofZoisite
CrystallographyOrthorhombic
Refractive Index1.692 - 1.700
Hardness6 - 7
Specific Gravity3.35
CleavagePerfect
Heat SensitivityYes
Wearability* Good
EnhancementsVirtually all tanzanite is heat treated
Special Care InstructionsAvoid rough treatment
FormulaCa2Al3(SiO4)3(OH)

Tanzanite Information

*Wearability is graded as Excellent, Very Good, Good, Poor, and Forget It! For more details see the article on “Hardness and Wearability.”

Truly a modern gemstone, transparent zoisite of a naturally reddish brown color which can be heated to a stable blue to violet, was discovered in the shadow of Mt. Kilamanjaro in 1969. Although other varieties of opaque zoisite were well known, they made no impact on the gem market. Tanzanite’s rise to prominence among retail jewelers and the general public has been rapid and dramatic. Naturally trichroic, the species shows different colors when viewed through each of its three crystal axes: blue, red-violet and yellow-green.

Although the occasional blue-violet stone is found in the rough state (Mother Nature in this case has already provided the heating); the majority of them must be heated to create this color. Usually stones are cut and polished prior to heating to about 700 degrees Fahrenheit, as abrasions, fractures and inclusions in the rough can cause cracking. This means that the cutter has to attempt to orient the stone for best color prior to the color change. It is the yellow-green color which is converted to deep blue by this treatment. Another important decision which must be made by the cutter is the choice to go for size (usually the more violet orientation yields the largest stone) or blueness (blue orientation yields smaller gems). A very small fraction of Tanzanite rough heat treats to a green or blue green color and such stones are valued by collectors.

Tanzanite Jewelry and Gemstone Information

.86 Carat Round Brilliant Unheated Tanzanite

In the trade, all Tanzanites are assumed to be heat treated and the color is stable. Initially, blue stones were valued as a substitute for sapphire, but gradually appreciation for the more violetish tones has risen. Tanzanite is used frequently as a ring stone, but with its hardness of 6.5, and it’s tendency to cleave, daily wear will dull the finish and its brittleness is a hazard. This lovely and expensive stone is better suited to earrings, pendants, tie pins and occasional wear rings or those with protective settings. Recent disastrous weather conditions, government embargos and continuing political tensions have restricted the supply, especially of larger fine colored stones.

Tanzanite Jewelry and Gemstone Information

Value

In general, stones showing more blue are valued higher than those showing more violet, and medium dark colors are the ideal. Custom cuts add value. As always, size and clarity have a strong effect on prices — large clean rough is extremely scare, and now in addition has been banned for export, so larger, fine gems are rapidly rising in price and decreasing in availability. Collector types such as greens or the ultra-rare cat’seye stones are highly sought after. Tanzanite Jewelry and Gemstone Information

Source/Attribution: Barbara Smigel at Artistic Colored Stones.

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