For Your Staff: Selling Treated Gemstones
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
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.
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.
- Gemstone Enhancement by Dr. Kurt Nassau, Butterworths,
- 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,
- Gemstone Enhancements: What You Should Know, American
Gem Trade Association, Dallas, TX.
- The Guide, Gemworld International Inc., Northbrook,
- Gemstone Buying Guide by Renée Newman. International
Jewelry Publications, Los Angeles, CA. 1998.