Welcome to AGTA GTC's Laboratory Update for October 25, 2005

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  1. Proletarian Gemology: The Blue Filter

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Proletarian Gemology: The blue filter

While I will be the first to admit that some gemological nuts require bomb-science technology to crack, others can be handled with the simplest of solutions – and a little bit of serendipity.
     Take the separation of natural and Verneuil synthetic yellow sapphire. This variety of corundum can sometimes be completely free of inclusions and because of the light color, color zoning is also difficult to locate.
     The formative years of my gemological career were spent in Bangkok, corundum capital of the world. And I was forced to test beaucoup quantities of the yellow stone, largely because dealers could find no gemological evidence with their 10x loupes.
     At that time, the technique of choice was immersion in di-iodomethane (methylene iodide). This is designed to eliminate surface reflection, thus allowing one to see the pattern of color zoning; straight or angular equals natural, while curved means synthetic.
     But even this technique is not without its problems. Placing a yellow stone into a yellow liquid over a yellowish light source is not an Einstein equation.

no blue filter

Figure 1. Mellow yellow
Two yellow sapphires immersed in di-iodomethane. Yellow stones in a yellow liquid over a yellow light make it extremely difficult to see color zoning. Photo: © Richard W. Hughes/RWH Publishing & Books

One day, I had a particularly nasty yellow sapphire in for testing. Absolutely clean internally, no iron lines in the spectrum. The only choice was immersion. After an hour of fruitless search for color zoning (and listing from side-to-side from the noxious fumes of an ever hotter immersion liquid), I decided to do a bit of experimenting.
     Several weeks prior I had purchased some white plastic filters for use with the microscope at a Bangkok sign-making shop. While there, I noticed they had frosted and clear plastics of virtually every hue; on a lark, I bought a selection of different colors.
     Thus on that day in the lab, I had filters close at hand and set about busily trying different colors beneath the immersion cell. Lo and behold, when I used a blue filter, the fog lifted and obvious curved color banding was staring me right in the face. Wow!
     A bit of experimentation with different stones showed me that, in order to see color zoning, the filter color should be the complementary color of the gem. With blue sapphires, immersing them in a yellow liquid (like di-iodomethane) is perfect by itself. Thus a white filter works great. For yellow and orange sapphires, a blue filter is best. And for ruby, a green filter does the trick.

Figure 2. Scream in blue
The same two yellow sapphires from Figure 1 immersed in di-iodomethane, but with a frosted blue filter beneath the immersion cell. Now the nature of the color zoning is extremely obvious. Photo: © Richard W. Hughes/RWH Publishing & Books

So there you have it – a simple technique – gemology for the common man. Indeed, it’s something even the bourgeoisie can appreciate.

The AGTA Gemological Testing Center provides the industry and the public with a complete range of lab services, including gemstone identification, origin determination and pearl identification. Located in New York City, the laboratory is equipped with the latest, technologically advanced, investigative equipment.The AGTA GTC is committed to providing excellent service, superior value and outstanding quality. A complete list of services and detailed pricing information is available on our website, www.agta-gtc.org. Please contact us with any questions.

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