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November 1999

Gemstones: Gemology

Testing for Tanzanite

A quick test to see if your "gem" is really a gem

We discussed the gemological properties of tanzanite and three of its simulants – synthetic forsterite, Coranite™ (synthetic corundum) and Tanavyte™ (synthetic garnet) – in our September 1999 issue (p. 48). We also promised a quick filter test you can use to help separate tanzanite from simulants. This month, we fulfill that promise.

This easy, inexpensive test is helpful as an initial separating tool when examining tanzanite parcels and when buying heated tanzanite at the source. Secondary gemological tests – such as magnification, determination of refractive index, short-wave fluorescence and specific gravity –  are needed on suspicious gems.

The quick test involves the use of an Aqua Filter,® a $12 filter invented and developed by Hanneman Gemological Instruments, Poulsbo, WA. The aquamarine filter was designed to separate aquamarine from similar but less-expensive gems such as topaz.

Here's how it works: Place the stones on a white background and illuminate with a strong incandescent light. Hanneman says fluorescent light will yield misleading readings. View the stones through an aquamarine filter and watch the gems change color. Certain colors indicate certain gemstones.

Here is a list of colors Hanneman says you'll often see by looking at certain gemstones through the filter:

Aquamarine

Turns green


Topaz

Turns blue gray


Spinel

Turns pinkish


Sapphire

Turns gray


Tanzanite

Turns pinkish orange


Zircon

Flashes of red, blue and green


Synthetic Forsterite

What about the new synthetic forsterite that looks similar to tanzanite? "It turns a bright green with the Aqua Filter®, " says Martin Haske of Adamas Gemological Laboratories, Brookline, MA, who sells the Russian-made simulant. Haske performed several tests at the Tucson gem and mineral shows this year and was the first to highlight the filter test.

Professional Jeweler decided to submit tanzanite and its simulants to the test, shown in the first photograph below. See if you can guess which of the stones are actually tanzanite. Give up? Look at the second photograph, and the explanation below.

by Robert Weldon, G.G.

Six gemstones photographed with the Hanneman Aqua Filter® and in incandescent light. Which ones are tanzanite?

The same unfiltered gemstones (from left): synthetic forsterite, Tanavyte,™ tanzanite, tanzanite, Coranite™ and synthetic forsterite.



Copyright © 1999 by Bond Communications.



 

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