Garnet with double refraction?

[Gempassion]

Hi,

I think that I was browsing a few Barry Bridgestock gems on one of the threads last week and someone mentioned that Barry said the garnet they had purchased from Barry displayed "double refraction". I think that "double refraction" was the term that was used (I am not sure... I tried searching the original post but I can't seem to find it). Does this means that all garnets are that sparkly or only some of them?

 

[PrecisionGem]

If a stone has double refraction, then it can't be a garnet. All garnets are single.

 

[Gempassion]

Thanks Gene,

I must have misread or misremembered.

 

[morecarats]

Some garnets exhibit a phenomenon known as "anomolous double refraction" (ADR) when examined with a polariscope.

For a long time it was believed that this phenomenon was indeed anomalous and did not indicate any degree of birefringence. More recent research has proposed that some garnets that form in metamorphic rocks undergo strain that deforms the crystal lattice and these garnets actually have a slight birefringence and this is what is being observed in what is called ADR.

From the point of view of scientific theory, it would be wrong to say that garnets are singly refractive as a matter of definition. The proposition that "All garnets are singly refractive" is an inductive conclusion, based on the observation of many garnets. It is no different than the proposition "No garnet is blue in color", which was once believed to be true and is now known to be false. If some doubly refractive garnets were discovered, it would simply change the definition of garnet.

 

[Gempassion]

In other words, the "anomolous double refraction" (ADR) of a garnet is anomolous because it is rare? Therefore, it is improbable but still possible?

 

[morecarats]

In other words, the "anomolous double refraction" (ADR) of a garnet is anomolous because it is rare? Therefore, it is improbable but still possible?

"Anomalous" doesn't mean rare, though anomalous results are usually rare. An anomalous result is an unexpected or contradictory result, such as when you're expecting a garnet to test as singly refractive and the instrument shows a faint indication of double refraction.

When confronted with an anomalous result, usually the theory or the measurement is flawed in some way.

It is possible that there are doubly refractive garnets, just as there are singly refractive zircons (metamict zircons which have lost their crystal structure as a result of radiation damage).

 

[PrecisionGem]

Morecarats, I'd be interested in seeing this new research. Do you have a source? I'm well aware of a false or ADR reading in a polariscope, but I haven't seen a garnet give two readings in a refractometer. Have you in your business? I'm assuming you are handling more stones than I am, as I think you are in the trade.

 

[morecarats]

Here are some references on the birefringent garnets:

http://www.minsocam.org/ammin/AM71/AM71_779.pdf

http://www.geo.uw.edu.pl/PTMINSP/2004/058.pdf

 

[Gempassion]

Thanks for the information!