VISUAL OPTICS- Special weapons & tactics for Guerilla Gemology

There’s a system devised by the Scottish gemologist Alan Hodgkinson (with some help from US gemologist Dr Bill Hanneman) which turns the eye into a human refractometer, capable of even outperforming the refractometer in some areas.

I use this system daily on the front lines, reducing by probably 70% the amount of times I have to reach for my refractometer or spectroscope.

This is the Reader’s Digest condensed version, but it will give you the general idea. For the in depth workbook, contact Alan at [email protected]. His website can be viewed at www.scotgem.demon.co.uk.

A diagram of how it works appears below. This and the other diagrams are posted with the permission of Alan, whom I contacted at his home/gem teaching school in Scotland. He gives short practical courses in all aspects of gemology, with his favorite topic being frugal advanced gemology using no or inexpensive instruments.

Right up the alley of Guerilla Gemology… For you non-professionals / non-gemologists, forgive me if I lose you once or twice in this article. If you’re a gem addict, I think you’ll enjoy it nonetheless.

As you can see from the attached diagram, the visual optics technique is based on using the pavilion facet angles of the test stone as prisms. You hold the stone’s table facet right up close to your eye until you can see through the stone. Usually the stone will be about to touch your eyelids. You then view a light source through the stone, preferably a small one (penlight, fiber optic), although you can use anything from the moon to a candle flame (which makes for a romantic setting).

The light enters through the pavilion facets, is spread into the rainbow spectrum (dispersion), bounces off the table and can be seen on the opposite pavilion facets as a “mini-spectrum”. In round stones you'll see multiple mini-spectrums goind around in a circle(s) according to the pavilion facet pattern arrangement.

On low refractive index stones you might observe 3 circular patterns, while a medium refractive index stone might exhibit 1 or 2, and a high refractive index stone only exhibiting the pattern if you tilt or lower the stone in relation to your eye. Very high refractive index stones (over 2.2) will not have circular patterns but rather “fields” of primary & secondary images which appear like snowflakes in and around your field of vision.

Several things to note about the mini-spectrum. The length of it will indicate the dispersiveness of the gem, how much it spreads the spectrum, resulting in “fire”. Leaded glass, for example, is highly dispersive and will have a long spectrum. Quartz has a low dispersion, with a short spectrum. This quality in itself is a helpful diagnostic.

Secondly, you will be able to tell if a gem is doubly refractive or singly refractive by whether or not the spectrum is “doubled”.

Thirdly, you will be able to guage the birefringence (how far the 2 rays are split) of the gem by how far apart the doubled images appear. Rotating a Polaroid filter between the stone and the light source further enhances this phenomena. A gem with high birefringence will show the two images completely separated, while a lower birefringence gem will show the two images touching, or even overlapping.

Fourth, This diagnostic feature is augmented by a B ratio table the author has compounded, which shows the ratio between the separation of the two spectrums (birefringence), divided by the length of the spectrum (dispersion).

Fifth, you can tell by how far away from the center of the stone that the pattern (referred to as the retina pattern) is reflected whether or not the stone has a low, medium or high refractive index. In fact, you can get results with the accuracy of a traditional refractometer if you combine this technique with a table top refractive index gauge which the author sells or which you can make yourself.

Sixth, you will often be able to spot major absorption patterns evident within the spectra. Not the fine stuff, you’ll need a spectroscope for that. But you can easily spot the difference between the spectrum of a ruby versus a garnet, for example. Both have clearly evident spectra, totally different from each other. This comes in handy for mounted stones which can’t be placed flush on a refractometer, among other things.

It also comes in handy when you’re out in the “field” having to make snap judgements about a particular piece you’re viewing.

Seventh, this technique can be a life saver when you become familiar with the “retina patterns” of diamond vs cubic zirconia vs moissanite. Each has a distinctive look, with the double refraction of moissanite providing an additional diagnostic.

Eighth, the symmetry and polish of gems can be somewhat evaluated with this technique (symmetry on gems with a refractive index of less than 2.2). The arrangement and spacing of the retina pattern will give you a clue as to the symmetry of the stone, while the crispness or blurriness of the image will give you a clue as to it’s polish.

Some examples of retina patterns and spectral images follow:

(Notice the doubled images of the zircon mini-spectrum)

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If nothing else, this technique is worth learning to differentiate between diamond, cubic zirconia, and moissanite.

Dealers, have you ever wondered whether cz’s or moissanites were mixed within a large parcel you were considering, but didn’t have the hours necessary to check each stone?

You can check hundreds of stones within minutes using a spin-off of the Visual Optics technique in the following manner:

Place the test stones in a glass dish, with water covering them. Turn the lights off and hold the dish an inch or two over a white sheet of paper with a flashlight shining down on them from above. Moissanites & cubic zirconias will instantly show up by their different pattern display and spectrum lengths compared to the diamonds.

You can also use this technique with a diamond as a calibration stone, next to a suspect stone. The difference in pattern display and spectrum length will be immediately noticeable.

Following are a couple examples of the differences using the Nelson Fingerprinter, and instrument devised to be used in conjunction with the Visual Optics system. These images are sharper and clearer than the results you will get with a flashlight, but the relative principle is the same.

Pretty cool, huh?

Here's a second attempt on the photo of diamond, cz & moissanite under the Nelson fingerprinter. The first one didn't seem to come out so well.

Richard,
THANK YOU ! I've been looking for this for several weeks and forgot where to go. Excellent post and thanks again.

No problemo, Mike. My pleasure.

Is that a pink tourmaline in your avatar?

Wow Rich...What an informative post... Thanks...

Richard,
That sure is a pink tourmaline in my avatar. My daughter picked it up as a very ugly pink/green oval, which had the most awful olive green overtones on the edges. I decided to try to cut out the olive colors and give it an unusual shape while I was at it, (she has a taste for one-off pieces). It worked pretty well, no green left.
I couldn't tell if you're a cutter, but even if you aren't the program "Gemcad" and its companion "GemrayX" are well worth the money, ($95), for their ability to take a gem design and evaluate it's properties without ever touching a lap. It's a very nice way to show a client what their poorly cut stone has the potential to look like.

Thanks for the GemCad and GemRayX tip, Mike. I'll put them on my (very long) list of wants.

That's the trouble with this business. You're always lusting for another gem or, in my case, another piece of equipment.

I think I went Guerilla by accident yesterday.

My finace and I were reading outside in the Park by my home(A very sunny Day)and we noticed when light reflected off her stone onto the white page at a certain angle we got all these bright white dots all over the page.(just like your example of a diamond above) I was like "Shiat" thats a lot of inclusions on her stone....then I rationalized that inlcusions would absorb light not reflect it.....

Of course these dots were brilliant white.....not colored

Thanks for the cool info Rich. I see the diamond retina pattern every day I drive the kids to school on the ceiling of the Suburban. The baby calls it "pretties!". Pretty neat effect. Thanks!

Spico and Daboysmom, in recognition of your frontline experiences, I award you with the highly esteemed Guerilla Gem Addict diploma.

Congratulations. That and $1.00 will get you a great cup of coffee at McDonald's.

Josh, seeing as you're well into postgraduate Guerilla Gem Addict territory, I will not only award you the diploma, but buy you the cup of coffee as well.

Just email me with the address of the McDonalds closest to you and I'll wire them the money.

Thanks Rich, but I was SO hoping for a Chai Tea Latte from Starbucks,,ho hum,,,off to carpool duty again,,,maybe the baby will buy me one?

I was at a show the other day when a dealer waved me over. He was looking at a "Thai ruby" another dealer was trying to sell him.

The way the stone was mounted the dealer couldn't get a refractometer reading on the stone, and it exhibited anomalous double refraction (a pattern under a polariscope which can resemble true double refraction). He was a little suspicious of the interior of the stone, but there wasn't enough inclusions for him to make a conclusive ID on the spot.

He asked me if I had my equipment with me to examine the stone, and I replied I did not. I then asked him to let me take a quick look at it.

After about 10 seconds using the Visual Optics method, I handed the stone back to him and pronounced a positive ID of "garnet".

Both he and the other dealer looked at me like I was David Copperfield, saying "Huh? How did you do that?". Heh heh heh. I love those kind of moments.

Using the visual optics method I was immediately able to determine that:

1. The stone did not have the characteristic ruby spectrum "signature".
2. The stone was singly refractive.
3. The stone had a significantly higher index than ruby.
4. The stone exhibited the strong "signature" absorption pattern of garnet.

As this stone was 4 carats in size, we're talking a significant amount of money for the ruby versus garnet gem ID. The dealer considering the stone was extremely grateful, while the selling dealer suddenly remembered another appointment he had to rush to.

Hi Rich,

I'm not sure if you have read too many comic books...LOL..

But you are a real life superhero who saves the day again...

Cheers to Rich for paving the way to honor and integrity amongst all the evil doers in the industry!

We love ya Rich

Does this kind of brown nosing get me breakfast with my cofee??

Excellent brown nosing job, Josh.

You can have the Deluxe Big Breakfast with your coffee.

This technique is so interesting.

I tried looking through the table as described, but I can't see anything. Is the ideal setup a darkened room with a small point of light? I just tried it at my desk with a olive oval tourmaline. I saw a bunch of green but no spectrum.

Can you give me a few more hints? With what stone is this effect most clearly seen?

Thanks so much for the valuable information!

Carter

A darkened room is helpful, although not necessary.

The key is a small source of light, like a penlight, or a flashlight whose lens is partially covered with black electrical tape, leaving a "slit" for the light to come through. This also serves to delineate the spectrum seen when looking through the stone.

You should be able to spot the retina pattern with a green tourmaline, or most any other colored stone. Lower refractive index stones have a "tighter, closer" pattern while higher refractive index stones throw the pattern out a bit.

Round stones are the best to start with, and peridot makes an excellent learning stone because of it's refractive index and high double refraction.

Green tourmaline has a distinctive pattern because of it's double images. They will appear right next to each other, with the outer image appearing weaker than the inner image. This characteristic is peculiar to tourmaline.

Thought you might like to hear of a different use for the Nelson Fingerprinter.

A tv Channel here in the UK use the Nelson Fingerprinter as a way of 'selling' moissanite to the consumer. They have a moissanite stone and a diamond side by side and turn off the lights to show the two patterns of coloured and white light. They then say 'look how much more sparkly the moissanite is compared to the diamond'.

Smart marketing...

I also notice that they've now started selling the strongly coloured green moissanite as a desirable stone in its own right. Nothing wrong with that either, IMHO, take your perceived weaknesses, double refraction and green tinge, and turn them into strengths instead.

Speaking of marketing, I heard a great account of how a smart executive dramatically increased the lackluster sales of red salmon when it first came out on the market.

He did it using five words on the label:

"Guaranteed not to turn pink"

Now if we could just get diamonds that carried a label that said

"Guaranteed to turn pink"

I'd buy one of those in a heartbeat!

Several years ago Mr. Hodgkinson was presenting at the Tucson show and said that if anyone was interested in coming to his house in Scotland for a few days that they should let him know.

I talked with him after the presentation and arranged to bring my daughter for an introductory course of gemology. We stayed at their home for nearly a week. My daughter learned a lot about gems and in the evenings we went for hikes along the beach and played at putting on the floor of his upstairs living room. (Wicked left break on the longer puts!)

Most humbling experiance. At the beginning of the course he invited me to take the initial stone identification test. No equipment allowed, just using your eyes and your fingers. As someone who "plays" with stones every day for a living I expected to do much better than I did! Sure, I got the easy ones, diamonds, CZ's, etc, but even some of the "name brand" stones are very difficult in their less common colors.

He taught my daughter how to use the equipment that we all rely on, but her favorite part was doing the Visual Optics part of his course.

After her course was completed Mr. Hodgkinson and his wife took us touring for a couple of days, what a great and friendly country Scottland is.

For anyone with a serious interest in gemology, he is one of the great all time instructors, and the gems you will see are alone worth the price of admission. His pricing is very reasonable and his hospitality is worth way more than he charges, even if he was not teaching any thing.

Wink

Wow, fascinating Wink!

I've been considering attending one of his practical gemology classes at his home Wyndhurst in Scotland, and your post has pushed me over the edge.

Thanks for the info!

Rich