Proletarian Gemology: The blue filter

Proletarian Gemology: The blue filter

By Richard W. Hughes


This article discusses the use of the blue filter to help identify yellow sapphires.


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.

Sapphire immersed in methylene iodide

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

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.

Sapphire immersed in methylene iodide with a blue filter

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

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

Sapphire immersed in methylene iodide

Blue filter
Figure 3. Po
wer to the people

The power of this technique is clear in the above two photos. Photos: © Richard W. Hughes


References & further reading

  • Hughes, R.W. (1987) Detection of color banding/growth zoning in natural and synthetic yellow/orange sapphires. ICA Lab Alert, No. 5, 1 pp.
  • Hughes, R.W. (1987) Identifying yellow sapphires – two important techniques. Transactions of the XXI International Gemmological Conference, MacGregor, I., ed., In International Gemmological Conference, Brazil, pp. 35–36.
  • Hughes, R.W. (1988) Identifying yellow sapphires – two important techniques. Journal of Gemmology, Vol. 21, No. 1, pp. 23–25.


About the author

Richard Hughes in Tibet
Richard Hughes
(left) is the author of the classic Ruby & Sapphire and over 100 articles on various aspects of gemology. Many of his writings can be found at


First published in October 2005, while I was at the AGTA GTC.




Views expressed in this article are the author's opinions alone and do not necessarily reflect the opinions of any organization that employs him. Those organizations bear no responsibility and assume no liability for content on this website, nor are they liable for mistakes or omissions.


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Posted 13 October, 2011; last updated 7 March, 2013