The skinny on cat's eye, color, Paraiba tourmaline

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The skinny on cat's eye, color, Paraiba tourmaline

By Mary L. Johnson, Ph.D.

January 11, 2009

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Mary Johnson has a doctorate in mineralogy and crystallography and is a former research and development manager at the Gemological Institute of America. Got gemological questions? E-mail them to national jeweler100@yahoo.com

Question: A mineralogy student asked (in the American Mineralogist forum):

"What's the cause of the 'milk-and-honey' effect in a cat's-eye? Much has been written describing the effect, but I couldn't find anything explaining how it occurs."

Answer: The same phenomenon occurs in fiber-optic cabochons or spheres. The light shines off the fiber- or hollow-tube walls when it is perpendicular to them; otherwise, the light is transmitted through the stone and so absorption causes the body color to be seen. Cat's-eye chrysoberyls are transparent "honey" color, and the hollow tubes of the stones together look translucent or opaque "milky" white.

Question: What kinds of promotions can jewelers set up with items from their vaults, especially loose colored stones?

Answer: Here are some ideas: Have a "Treasures from our Vault" case, set up like an exhibit with a discreet note indicating that items in the case are for sale.

Rotate items on a regular basis and invite customers to visit and see what has changed in the case.

Often, the only things in a jeweler's cases and displays are saleable goods; but this approach may not attract every customer's eye. In contrast, consider Tiffany's famous windows, in which one jewelry piece takes the spotlight but other kinds of materials dress the set. With this in mind, consider a "safe-cracker sale," with a decor of miniature "Keystone Cops" and striped-pajama-style convicts with black masks.

Do you have a local art school? Hold a design contest and let your bench jeweler be a judge to make sure the winning design is practical. If possible, the winning piece should be made and then exhibited. Don't forget to send out press releases to local media.

Finally, keep in mind that "everything old is new again." With the high prices of gold and platinum, new styles for the coming year include chunky silver-and-gemstone jewelry and "love beads" strung on unusual materials. So perhaps the treasures in your vault will inspire you as well.

Question: Can I describe all blue color tourmaline from Africa as "Paraiiba tourmaline?"

Answer: The short answer is "no." Even in Brazil, not all blue tourmalines are "Paraiba tourmalines" and not all Paraiba tourmalines are blue (they can be green or purple as well). The original find of tourmalines in Paraiba owed their colors to copper (sometimes in addition to manganese).

Recently, CIBJO decided that any blue-to-green tourmaline colored by copper could be designated as Paraiba tourmaline. (See also http://www.palagems.com/paraiba.htm). However, most blue tourmalines are not colored by copper, owing their color instead to iron and other elements. It is never proper to refer to these tourmalines that do not contain copper as Paraiba tourmalines.

To identify copper-bearing tourmaline, try to get an EDXRF (energy-dispersive X-ray fluorescence) analysis, which will distinguish whether or not copper is present in the tourmaline. LA-ICP-MS (laser ablation inductively-coupled plasma mass spectrometry) should be able to distinguish among specific sources, such as Brazil, Nigeria and Mozambique. Several gemological laboratories have access to these devices.

Editor's note: Mary Johnson's "Gemological Answers" column first appeared in the September 2008 print edition of National Jeweler, in the Your Store section.

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