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Identifying Forsterite From Tanzanite

Gems
By Stuart Robertson, G.G.
This story appears in the September 2005 issue of JCK magazine
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Tanzanite is one of the most popular gems of the last 20 years. In its finer qualities, tanzanite resembles blue sapphire. And at the height of production in the early 1990s, a wide range of qualities and sizes was available. The stone gained acceptance in its own right as dealers no longer saw it merely as a less-expensive alternative to fine blue sapphire. More recently, tanzanite production has declined sharply. However, its popularity (especially in Caribbean tourist markets) has created the perfect environment for imitation tanzanite to flourish.

One such product is synthetic forsterite. It has been available for several years, but sale of this imitation has jumped during the past six months. JCK has encountered more forsterite this year than in the previous five combined. Recently, an oval cushion-cut stone was submitted for identification. The stone was purchased as fine tanzanite by a consumer touring Tanzania.

To the unaided eye, the stone resembled tanzanite. It exhibited a strong pleochroism. Observation through a GIA calcite dichroscope revealed purple and blue colors. Although the purple was consistent with the purple seen in tanzanite, the blue was more reminiscent of fine blue zircon. Standard gemological measuring and testing provided the following results: 9.32 mm × 8.79 mm × 6.20 mm, with a weight of 4.575 cts. Examination at 60× revealed numerous white pinpointlike inclusions. The sheer number suggested synthetic origin.

Using a standard GIA Duplex II refractometer, a refractive index (RI) of 1.635–1.647 was obtained. This is much lower than the 1.690–1.700 RI of tanzanite. The stone was then examined through a Hanneman tanzanite filter. The colors purple and blue, along with a diagnostic green, were observed through the three windows of the filter. This confirmed the suspicion that the material was synthetic forsterite, since tanzanite would be expected to produce a pinkish orange color through the filter in addition to purple and blue.

It is important to follow the instructions for using the Hanneman filter exactly. Turn off any overhead fluorescent lights. Illuminate the stone from the side with a bright pen light or fiber- optics source. Placing the stone table down on a white paper background will help to discern the color observed through the filter.

Special thanks to the Morion Company, Brighton, Mass., (617) 787-2133, for supplying synthetic forsterite, and to Emerald House, New York, (212) 921-2222, for supplying the tanzanite.

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