
Proletarian Gemology: The blue filter
By Richard W. Hughes
Abstract
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.

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.

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.


Figure
3. Power 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 (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 www.ruby-sapphire.com.
Notes
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
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