A frica produces many unusual gemstones as well as many traditional ones. One—from Tanzania—has evolved from the unusual to the traditional. The gem is tanzanite, a transparent blue-purple variety of the mineral species zoisite. Its richness of color is comparable to that of the finest Burmese and Ceylon sapphires.

Opaque zoisite has been used as an ornamental gem, both on its own (as in the variety thulite, a pink Norwegian stone) or as a green material in combination with opaque ruby, also found in Tanzania. The latter commonly is seen in ruby-zoisite objet d’art carvings.

But tanzanite is a gem of a different color. Although it was discovered little more than three decades ago, many believe its impact on the colored gemstone market has been greater than that of any other gem in recent history. In fact, tanzanite has edged out emerald as one of the three top-selling colored gems in U.S. retail jewelry stores.

History and romance. Tanzanite was discovered in Tanzania in 1967. According to historical accounts, nomadic Masai tribesmen, cattle herders of southern Kenya and northern Tanzania, first stumbled across the gem, which started out as the more common brown zoisite but turned blue after being engulfed by annual brush fires. These blue zoisites were nature’s heat-treated tanzanites.

The Masai led Portuguese prospector/geologist Manuel de Souza to the find. De Souza, the first outsider to recognize the gemstone’s potential, took the zoisites to Germany, the blue ones for cutting, the brown ones for heating. In Germany, he ran into Harry Platt, chairman of Tiffany & Co., who was on a diamond buying trip. Tiffany introduced tanzanite to U.S. consumers in 1969 at its newly opened San Francisco store. Tiffany also was responsible for naming the new gem, after its country of origin.

Tanzanite is found in a single commercially mined area in the Meralani hills, a two-hour drive southeast from the capital, Arusha. En route, you can see Mount Kilimanjaro looming in the distance to the northeast. Unfortunately, tanzanite mines remain primitive, which can lead to disaster. Floods last spring caused mines to collapse, resulting in scores of deaths.

Color variations. Natural zoisite crystals, typically a slightly reddish-brown or beer-bottle color, are heated to

600-650°C to change the color to the beautiful trademark blue-purple of tanzanite. Bluish-purplish tanzanite does occur in nature, but much of it is weakly saturated and still requires heat treatment to improve the color. Thus, a large percentage of tanzanites—some would say all—are heated, and it’s nearly impossible to find an untreated one.

Qualities. Hue, which can vary from pure blue to pure purple (and every blue-purple combination in between), is the most important consideration in choosing a tanzanite. But look carefully. Tanzanite’s color can shift under different light sources. What appears to be pure blue outside might look purple inside. Be especially attentive to matching stones. Bluer tanzanites tend to be more expensive because more weight loss occurs when the crystal is cut to show blue color face-up. Since tanzanite typically is found in eye-clean crystals, clarity generally isn’t a factor.

Low-quality tanzanites—small stones of pale-blue, purple, or gray—are readily available and are sold on television shopping channels and at high-volume jewelers. The supply of high-quality gems fluctuates; last year’s mine floods temporarily disrupted supplies and sent prices higher.

Pricing. Tanzanite prices are up about 20% over last year, to $350 to $475 per carat for fine-quality 1-ct. to 2-ct. goods. Prices could go even higher as demand outpaces supply.

Care and cleaning. Tanzanite, with a hardness of 6.5 on the Mohs scale, is less durable than quartz, a category of minerals that includes citrines and amethysts, both rated 7 in hardness. Since it’s easily scratched, abraded, and chipped, tanzanite should be reserved for evening wear or moderately active daytime use.

Tanzanite is best set into earrings and pendants that have little chance of getting knocked around. When setting it into rings or bracelets, pick a mounting that will protect the stones as much as possible. Of primary importance is how consumers care for tanzanite jewelry when it’s not being worn. Storage in jewelry pouches with individual pockets is recommended.

When cleaning tanzanites, remember that ordinary dust, which contains quartz, can easily scratch them. Use warm, soapy water to remove dust before wiping the gem clean. Do not use an ultrasonic cleaner.

Bench settings and precautions. Before repairing tanzanite jewelry, remove stones from their mountings. The heat of a torch not only can change the gem’s color but also can break the stone.

There are a number of tanzanite lookalikes. The Lannyte Co. in Houston, which promotes lab-grown and simulated gem materials, offers two. “Coranite,” a lab-grown corundum (hardness, 9), is blue with a hint of purple; “Tanavyte,” a lab-grown garnet-based material (hardness, 8.5; refractive index, 1.85; bright orange under UV), is purple with a hint of blue. The latest tanzanite lookalike comes from Morion Co. in Brighton, Mass., which is selling synthetic forsterite, a chromium-rich mineral (hardness, 7) grown in Russia.

Recommended reading. For more information, see the following references:

N.R. Barot and Edward Boehm, “Gem Quality Green Zoisite,” Gems & Gemology, Spring 1992, p. 4.

D.M. Dirlam, E.B. Misiorowski, R. Tozer, K.B. Stark, and A.M. Bassett, “Gem Wealth of Tanzania,” Gems & Gemology, Summer 1992, p. 80.

Joseph Purtell, The Tiffany Touch, (Random House, 1971).