Tabby extinction!

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PinkDiamond
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Tabby extinction!

Post by PinkDiamond »

We have some information on ADR, or 'tabby extinction' on the forum, but this gives more information and has new .gifs to show you exactly what it is, and how it differs from what you'll see under a polariscope in a doubly refractive stone. Image :mrgreen:

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a publication of the Insurance Institute of Jewelry Appraisal, a 501(c)3 Non-Profit Education Organization Appraisal 7 January 2019

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All of my fellow cat lovers out there....calm down and purr on! Cats are not being driven to extinction by dangerous predators such as dogs, automobiles or Alexandria Ocasio-Cortez. All is well in the feline world. What we are talking about is "tabby extinction," technically known as Anomalous Double Refraction or ADR. You know, the reaction you get when you think you are seeing double refraction in your polariscope but then you think you aren't, then you think you are, then you think.....you get the point.

Today we are going to look at real ADR to help you identify the real thing when you see it.

What is ADR?

ADR happens in certain single refractive gemstones where the crystal structure causes an unusual dark to light reaction that normally identifies a double refractive gemstone. The result, particularly for new gemologists, is a lot of head-scratching and hand-wringing because the refractometer gives a single RI reading of a single refractive gemstone...but the polariscope gives the dark to light reaction of a double refractive gemstone. Or does it?

True Double Refraction in a Polariscope

Many of you will remember the working polariscope we made last spring using a pair of polarized sunglasses. I thought it would be fun to pull this slide show out for our demonstration of true double refraction in a polariscope...even if it is a hand-made polariscope from sunglasses.

(Thank you, Dr. Hanneman and Alan Hodgkinson for teaching us that viable gemology tools need not be expensive or from big manufacturers)

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As you can see at left, this blue topaz in our polariscope turns completely dark to light as the stone is turned between the polarizing filters.

This is total extinction and is diagnostic for a double refractive gemstone.

But what if you get something like the next image below...

Anonymous Double Refraction

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At left is another slide presentation of a garnet being turned between the polarizing filters of a desk top polariscope. If you look closely what appears to be the same dark to light extinction seen above is actually not a total extinction of the light at all.

If you look closely the dark to light extinction is in-line patterns that never fully extinguishes the light. This garnet is giving us anomalous double refraction and is clearly demonstrating what we should be wary of when looking at many garnets through the polariscope.

But where did the term "tabby extinction" come from? Let's look at the next panel.

"Tabby Extinction"

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As you watch this 10x magnified view of a lab created spinel in a desk top polariscope you will see the reason for the name. The lines in the fur of a tabby cat very closely emulate the ADR lines shown by this created spinel. As it rotates between the polarizing filters the lines move and reshape themselves.

New gemologists who only take a quick look and see the light to dark changes very often mis-identify this as double refraction. However, with a little patience and close inspection it is easy to see that this is indeed ADR or tabby extinction, and not true double refraction.

For the experienced gemologists out there, this may seem a rather simple issue. But for new gemologists just getting experience in gemstone testing, the above can save you hours of searching through textbooks, gem charts and Google trying to find a gemstone that is double refractive in the polariscope but single refractive in the refractometer. In truth, there is no such thing. What you see is caused by Anomalous Double Refraction.

Sometimes referred to as "tabby extinction".

Maybe that name was coined by someone who hates cats. I hope not. I love cats.

But not as much as I love gemology.

Robert James FGA, GG
President, Insurance Institute of Jewelry Appraisal Inc.
a 501(c)3 Non-Profit Education Organization
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PinkDiamond
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