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 Post subject: Garnet looks DR on Polariscope
PostPosted: Wed Apr 12, 2006 2:20 am 
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First, I'm a newbie who has taken the 1st GIA distance course (Colored Stone Essentials) and an Introduction to Gemology course at the local university. The text we are using is EXCELLENT and goes into a lot of theory: Gemmology by Peter Read, FGA. Plus, I'm doing a lot of reading and self-education and continuing my GIA studies.

That said, I just got my first Polariscope and decided to go thru my collection of gems, starting with Garnets (my favorite gemstones) to show they were not doubly refractive, and cubic, by inference.

I was blown away when I saw most of them go through the LIGHT-DARK sequence 4X in 360 degrees, as if they were DR. These were definite light/dark sequences of the whole pavillion of the stones. This included Mandarin spessartites and umbalites (combo of pyrope & almondine w/some spessartite). My Pyropes stayed dark on the polariscope.

These garnets are all from a lot of different sources, mainly Thai dealers over Ebay -- the higher volume more well-known dealers. Most of the special stones were hand cut in Czech Republic by, who I believe to be a reputable dealer.

But back to the testing:

SG and RI
I have not tested SG (I don't have a way to do it), nor have I tested RI. I would say that many of these stones, if garnet, would be off the RI scale. But it is a test I plan to do.

DICHROSCOPE
I couldn't see any difference in color on the 2 little squares for any of these stones

"VISUAL OPTICS"
1. I tried to see if the stones had facet doubling. Didn't look that way, but my tired eyes were not sure (if no doubling, probably SR)
2. I tried to see if I could see a double rainbow looking through table of the stones and they appeared to have single rainbows (if so, SR)

At this point, I am totally confused as to why these "garnets" (if that's what they are) are behaving in this way. It's hard to believe so many garnets from so many dealers (including some TV shopping networks) are not authentic. I must be doing something wrong.

Can anyone suggest anything?

Thank you so much.


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PostPosted: Wed Apr 12, 2006 8:34 am 
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Welcome to our forum nitedawg!

When you get to GIA's Gem Identification course, you will discover that garnets are famous for showing ADR on the polariscope. Red, purple, and orange stones often show ADR on the polariscope, so other tests are needed to positively ID them.

Try this to see if they are indeed SR: take a white business card and poke a very small hole in the center. Put your stone on the polariscope and turn the stone where it appears to be the brightest. Place the hole over your stone on the polariscope. Turn the polarizing filter while looking through the hole. If the stone is SR - you will see the stone get much brighter. This test is not infallible however as red, purple, and orange stones can play tricks on you in the polariscope.

Garnets have definate inclusions that can be used for identifying them. Specific gravity tests are also helpful. The spectroscope can often positively identify garnets.


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PostPosted: Wed Apr 12, 2006 10:04 am 
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Welcome Nitedawg.
Just in case anyone doesn't know, ADR stands for anomalous double refraction


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PostPosted: Wed Apr 12, 2006 12:06 pm 
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Welcome, Nitedawg!

Just in case anyone doesn't know, anomalous is an adjective that means: Deviating from the common rule; irregular; exceptional; abnormal. :)


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PostPosted: Wed Apr 12, 2006 2:27 pm 
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I agree with all above about ADR in garnets. In fact, some garnets formed under extreme stress are actually very slightly birefringent (see "American Mineralogist," vol. 83, pp 1293-1301, 1998).

The degree of birefringence is directly related to the amount of stress encountered during tectonic deformation, with mantle garnets from kimberlites having the highest values. They are not usually encountered, though, so a quick Visual Optics check is usually all that's needed to establish that a stone is SR.

I disagree that S.G. is a reliable property in distinguishing garnets. Values vary a lot depending on the type/amount of inclusions in any individual garnet.

The only garnet ID scheme that makes sense to me (barring access to my own personal spectrophotometer) is the one outlined by W. Wm. Hanneman Ph.D. in his book "Naming Gem Garnets." It's based on color (or spectrum) and R.I. only. The only problem using it is determining R.I. exceeding the 1.81 limit but he offers his Garnet Refractometer as a solution. It's a pinhole refractometer made, origami-like, from a paper fold-up that comes with the book. It works with cut stones only and involves a bit of a learning curve, including some method of determing the angle of a stone's pavilion main facet.

I've studied garnets pretty intensively for the past 7 years. They are not easy to understand, especially since what is taught by GIA is at great odds with other gemologists and gemological institutions around the world.
Garnet value is presently too low to demand the kind of scientific attention required to settle a lot of conflicting classfication issues.

ROM gg (gonzo gemologist)


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PostPosted: Wed Apr 12, 2006 2:48 pm 
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I came across an interesting paragraph on Garnets and ADR in Webster's Gems, 5th Edition, page 193:
Quote:
Although garnets are, from their crystal structure, singly refractive, many garnets display local double refraction when examined between crossed polars. Usually this ADR is confined to dark streaks, often in 2 directions at right angles, but some garnets show a convincing "four times light" and "four times dark", during a complete revolution on the stage. This optical anomoly is ascribed to stress due to the isomorphism, but as idocrase, a tetragonal mineral, has an atomic configuration close to garnet, it might well be that at least some garnets are tetragonal in structure.


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PostPosted: Wed Apr 12, 2006 3:57 pm 
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ROM wrote:

I disagree that S.G. is a reliable property in distinguishing garnets. Values vary a lot depending on the type/amount of inclusions in any individual garnet.


Perhaps I need to clarify - I was not implying that garnets could be separated from each other with S.G. testing - I was implying that it can be helpful (I did not use the word "reliable") in separating garnets from other stones.


Last edited by Snizzy on Wed Apr 12, 2006 3:58 pm, edited 1 time in total.

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PostPosted: Wed Apr 12, 2006 3:57 pm 
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Hi Barbra,

That IS fascinating. The article I cited earlier mentions well documented optical anomalies in the ugrandites. My reading indicates that stress may induce some internal crystallographic changes. But how and what they are is beyond my complete understanding.

They make statements like: "For this solid solution we hypothesize that the internal strain primarily arises in structural variations of the tetrahedral site, derived from its unusually long bond lengths and irregular shape...Thus the hydrogarnet substitution in the tetrahedra could play a crucial role [in birefringence] or it simply could be a passive marker of the [stress] distortions."

I have a number of garnets from Orissa, India that show distinct 4 light, 4 dark positions on the polariscope. They're all pyrope-almandines. I'll have to play with some other species when time allows.

ROM


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PostPosted: Wed Apr 12, 2006 4:09 pm 
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Moderator wrote:
Perhaps I need to clarify - I was not implying that garnets could be separated from each other with S.G. testing - I was implying that it can be helpful (I did not use the word "reliable") in separating garnets from other stones.


I didn't mean to come off so emphatically. :oops: Yes, I agree S.G. can be helpful in the way you suggest. I'm so used to thinking in Hanneman terms where its use is excluded (garnet vs. garnet) it didn't occur to me to to relate it to other stones in that context.

ROM


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PostPosted: Wed Apr 12, 2006 4:20 pm 
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LOL, ROM! :lol:

Since nitedawg was wondering if he/she had actually received garnets - I was just suggesting that a spectrascope and an SG test might help make a positive identification, where the refractometer and the polariscope were lacking. :wink:


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PostPosted: Mon Apr 24, 2006 7:09 pm 
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Sorry for not having responded to all of your great suggestions and feedbacks - many thanks for your interest in the problem I've found myself with.

Barbra, within your quote is exactly what Ive found: A definite set of 4 light/dark sequences over 360 degrees. As to it being Idocrase, the RI that I found below (1.758) is higher than Idocrase (1.700 - 1.723).

"Although garnets are, from their crystal structure, singly refractive, many garnets display local double refraction when examined between crossed polars. Usually this ADR is confined to dark streaks, often in 2 directions at right angles, but some garnets show a convincing "four times light" and "four times dark", during a complete revolution on the stage. This optical anomoly is ascribed to stress due to the isomorphism, but as idocrase, a tetragonal mineral, has an atomic configuration close to garnet, it might well be that at least some garnets are tetragonal in structure."

And ROM, your statement:
"I have a number of garnets from Orissa, India that show distinct 4 light, 4 dark positions on the polariscope. They're all pyrope-almandines. I'll have to play with some other species when time allows. "

This also rings true with me because of the 4 light-dark changes BUT also because it happens mostly in my UMBALITE garnets, that is the ones supposedly) from the Umba river valley in Tanzania - as I understand it, Umbalites are a combination of Pyrope and Almandine with traces of Spessartite.

RI
I tested a large umbalite that has perfect 4X light-dark sequences and its RI is 1.758 (the 8 is approximate)

SG
Haven't tested this yet -- just got a little SG scale contraption to attach to my carat scale and need to play with it. Then I'll take the metric of my garnet (or probably of a few).

I look forward to hearing from members here on further suggestions or feedback -- I AM a rank amateur just starting out on this adventure into Gemology.


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 Post subject: Garnets and other red stones
PostPosted: Tue Jul 25, 2006 5:50 pm 
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Hello!
I am a new member of the forum, and a very new gem enthusiast. I'm trying to learn more about gem identification, but I do not have all of the expensive equiptment yet. Obviously, the ID of some stones is easy to determine visually. However, red stones are particularly troublesome for me. Any information on how to distinguish garnets from other similar stones would be greatly appreciated.

I purchased a portable polarimeter, but this has not been helpful, since it arrived with no usage instructions. A web search revealed several conflicting sets of instructions. I've tried rotating the stone (as one site suggested), and I've tried rotating the top lens (as another site suggested). In both cases, nothing seemed to happen to the stones, but the light itself went light/dark. I've tried to compare a garnet (for SR) with a same-size andesine/labradorite (for DR), hoping that any differences would give me insight. I had no luck with this.

What am I doing wrong? Do portable polariscopes really work, or will I need to invest in a desk model? Would a dichroscope be a better device for separating SR from DR?

Thnks for the helpful forum! :D

Melanie


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PostPosted: Wed Jul 26, 2006 7:10 am 
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Hi Melanie,

I take it you mean the polariscope.

"Cross" the two polarization filters. That means that if you look through them, no light can pass (dark). Then hold the stone in between with your fingers (making sure the light can pass through the stone and not your fingers).
Then laterally turn the stone 90 degrees left and right.

If the stone stays dark, it is SR. If it goes from light to dark, it's DR.

So only turn the stone, not the filters themself.

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Proud to be a DSN and JTV shopper, just love the guys!


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PostPosted: Wed Jul 26, 2006 9:48 am 
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Hi Melanie ... Welcome aboard!

I have a portable polariscope and they work just fine. What no one probably told you is that for some reason (I still don't quite understand why) the polariscope is useless on red stones. Personally, I would use a dichroscope on them. I'm sure other, more experienced, folks will chime in soon to help you out :D .


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PostPosted: Wed Jul 26, 2006 10:54 am 
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Hi Melanie.
What kind of light source does this portable polariscope use? Is it internal or external?
:idea: Can you take a digital picture of it an upload it to the forum?

As Doos suggested make sure you are rotating the polars so that it is completely black when you look down through them. At this point, put your gem between them. I actually rest my gem, table down, on the bottom polar and slowly turn it around, nudging it with my finger. Then I rest it on a pavillion facet and turn it around again. Don't apply vertical pressure, as you don't want to scratch your polars.

I don't use tweezers unless I'm looking for an optic figure.


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