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September 1999

For Your Staff: Selling Treated Gemstones

Treated Tanzanite

Gentle heating transforms brownish zoisite into the striking purplish blue tanzanite that has taken consumers by storm in the past few years

The deep blue tinged with violet you've come to associate with tanzanite is rarely the natural color. Though some natural blue and violet tanzanites exist, the vast majority are brown or orangy-brown when mined. Gentle heating transforms these crystals into the more marketable colors.

Because we rarely – if ever – see the natural-color version of the gem in the marketplace, we tend not to think of tanzanite as being enhanced. In addition, tanzanite isn't associated with processes such as fracture filling, dyeing or irradiation. Yet those pretty purples and beautiful blues almost always result from heating.

Generally, the crystals are preformed, heated to a uniform 350°-600°C (depending on size) then faceted. When heated, tanzanite loses its yellow component, leaving behind purple and blue. When you examine tanzanite, you see blue, pink or purple – depending on how you look at it. The reason is that tanzanite is trichroic, meaning it exhibits three colors because of three crystallographic axes. The color you see through the table results from how the tanzanite was cut.

Introducing Enhancements
Show your customer a range of tanzanite shades and explain the differences result from size, intensity and cut. You can tell your customer tanzanite, known mineralogically as zoisite, is almost always brownish when removed from the earth. Explain the majestic purplish blue results from human ingenuity in gently heating the gemstone.

You might also take this opportunity to point out tanzanite can be treated in this manner because it is relatively inclusion-free (inclusions expand at a different rate than the gem when heated, causing stress fractures).

Special Care Warnings
Tanzanite is 6.5 on the Mohs hardness scale, making it softer than gemstones such as topaz and tourmaline. It can be used in rings, but the wearer should be careful not to knock it or drag it against a rough surface. Heating does not affect durability. The color won't fade over time or with exposure to light, therefore, it's considered stable. To clean tanzanite jewelry, use a soft brush and lukewarm, sudsy water.

Advice for Sales Associates
Learn your store's disclosure and return policies regarding tanzanite. Your store may want to adopt a written policy for gems of this kind that a customer can read, understand and sign.

Recommended Reading

  • Gemstone Enhancement by Dr. Kurt Nassau, Butterworths, London, England.
  • Gem Identification Made Easy by Antoinette Matlins, Gemstone Press, Woodstock, VT.
  • American Gem Trade Association Source Directory 1997/1998 (contains Gem Enhancement Manual). American Gem Trade Association, Dallas, TX.
  • Gemstone Enhancements: What You Should Know, American Gem Trade Association, Dallas, TX.
  • The Guide, Gemworld International Inc., Northbrook, IL.
  • Gemstone Buying Guide by Renée Newman. International Jewelry Publications, Los Angeles, CA. 1998.

The bluish purple in the crystal at right is natural – very rare for this gem variety. The vast majority of tanzanites, like the cushion-cut 5.19-ct. gem at left, are heated to achieve the purplish blue or blue. The crystal and cut gemstone are courtesy of Pala International, Fallbrook, CA.

– by Robert Weldon, G.G.

Copyright © 1999 by Bond Communications.


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