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 Post subject: Puzzling Red Garnet?
PostPosted: Tue Jul 12, 2011 4:20 am 
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I have been worrying over the the identification of a small red gemstone for several weeks and I shall tell you how it came about. While I was writing up an article for the popular press on the use of magnetism to test gemstones for Helium.

I found a packet labeled red garnets which I hadn't seen for decades. It contained about 20 faceted stones ranging from tiny rose cuts, a marque and several ovals up to 3 carats. So I got out my trusty heavy duty magnetic wand to test them. I could pick up them all except one oval. Ho Ho, I said, what do we have here????

Not a problem, I thought, probably paste, altho it was a brighter red than the other almandine garnets. I shall measure the RI. which turned out to be 1.778 (checked several times). This is in the almandine range, in fact the other garnets had an RI of 1.773 to 1.775 which indicates about 60% almandine "molecule" in the pyrope-almandine series.

So why is it non-magnetic? Is it? I tried the flotation method where you put the gem on a cork in a bowl of water and tested with the magnet. Not a movement, whereas the other garnets went berserk, or could be picked up.
So there is something funny here.
I examined the gemstone in more detail:
Data: Red colored oval size 8.2 x 6.1 depth 4.1 mm, wt 1.90 carat.
Polariscope: Isotropic as were the associated almandine gemstones. No dichroism.

Magnification: At 10, 30, 45 x I checked out the stone; plus similar size garnets: Natural type inclusions present including incipient fractures, triangular flat thin plates, and in some areas many clusters of tiny needles in parallel orientation, intersecting at maybe 60 degrees. This convinced me that it was a natural gemstone and not paste.

Chelsea filter: The almandine garnets remained dark or greenish, but this puzzling stone showed glimpses of red, suggesting that it transmits more red than a normal garnet.

Fluorescence: Inert to UV and blue radiation.

Spectroscope: (Using prism & diffraction spectroscope) The almandine garnets showed their typical 3 strong band absorption spectrum. My puzzling gemstone showed only one strong band peaking around 550nm, or it freely transmits the blue and most of the red and orange. No sign of any Cr lines in the red part. Very peculiar. I don't know what to make of it all.

Is it possible to have an almandine type garnet with a combination of ferrous/ferric and other chromophores that negates magnetism? I don't know but would be interest in any feedback on the matter.
Cheers
Allano
PS: I was going to add a foto but can't see how to do it... I must be going stupid in my old age.


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 Post subject: Re: Puzzling Red Garnet?
PostPosted: Tue Jul 12, 2011 10:39 am 
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Perhaps Kirk Feral could weigh in on this question.
See his extensive article on magnetism and gems here:
http://gemstonemagnetism.com/overview_o ... tones.html


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 Post subject: Re: Puzzling Red Garnet?
PostPosted: Tue Jul 12, 2011 11:43 am 
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From my understanding there is nothing that would so drastically counteract the magnetic capabilities of the chromophores in a natural almandine garnet that could completely and exactly negate a reaction from this stone. That being said, there are always exceptions. However with all the other unusual measurements you took from this stone compared to the other specimens in the packet, I would think that it was in fact not a natural garnet...although I admit I can't seem to come up with another suggestion as to what it might be that fits all your data.

The only thing I could figure would possibly be a very red Rhodochrosite with a slightly off RI, perhaps? Normally I think this would show a good magnetic reaction, but the rosier the color the less iron content and therefor the weaker the reaction to the magnet, so perhaps this could account for the lack of magnetism. This is quite a long shot though, and currently I can't check all the other data you measured against known Rhodochrosite reactions, but it's all I can come up with off the top of my head unless you had a very, very strange Ruby.

Sorry I can't be of more help, but I can say for sure that if it was natural almandine garnet it should show a magnetic reaction of some sort.


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 Post subject: Re: Puzzling Red Garnet?
PostPosted: Tue Jul 12, 2011 2:59 pm 
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Thanks for your suggestions.
I have pondered the problem overnight and to reconcile all the tests results (more or less) points to one likely possibility which I shall have to check out. It will mean redoing some tests more carefully and to use lateral thinking!
I'll let you know the result when I am happy it is correct.
Maybe you can guess what I am thinking?
Cheers
Allano


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 Post subject: Re: Puzzling Red Garnet?
PostPosted: Tue Jul 12, 2011 3:21 pm 
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High magnetic susceptibility is a defining characteristic of Almandine Garnet, and any Almandine that size will pick up with an N-52 magnet. Rhodocrosite is colored by manganese, and is even more magnetic than Almandine. My guess is the non-responsive gem in your parcel of Garnets is a Ruby. The bright red color and Chelsea filter reaction suggest this. Almandine and Corunudm RI's can overlap. You may be seeing corundum "silk" rutile needles under the microscope. Polariscope and dichroscope readings can sometimes be unclear. Did you check for bi-refringence when you took your RI reading?
Kirk Feral


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 Post subject: Re: Puzzling Red Garnet?
PostPosted: Tue Jul 12, 2011 7:11 pm 
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Thanks Kirk for your input. Yes it might be a ruby so I read up about rubies. When in doubt read Anderson's book on "Gem Testing", but you have to get the right page.

My overnight thought was the only thing that it might be was a garnet topped doublet of red paste, imitating not garnet but ruby. So I read up about ruby imitations in Anderson.
He says ... "Red doublets composed of a thin slice of almandine garnet serving as the table facet fused to a red glass base, are still sometimes seen. The table facet gives an RI reading of about 1.79 and the back facets about 1.63...etc the join should be visible when viewed immersed in a liquid etc

So back to the refractometer to measure RI on a back facet ...yes I get a result of 1.63. I shall do more testing later.
Of course an SG estimate with bromoform would have been helpful but I did not have any available. Stone must be a relic or a 100 years old. It now gets put in my packet of few doublets.
Mystery solved I think.
Allano


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 Post subject: Re: Puzzling Red Garnet?
PostPosted: Wed Jul 13, 2011 11:49 am 
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Allano, sounds like you may have a ruby/glass doublet. A garnet/glass doublet made with an Almandine crown would be easily detected with an N-52 magnet, as the crown side would drag, while the pavilion would not.


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 Post subject: Re: Puzzling Red Garnet?
PostPosted: Wed Jul 13, 2011 12:03 pm 
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Kirk Feral wrote:
Allano, sounds like you may have a ruby/glass doublet. A garnet/glass doublet made with an Almandine crown would be easily detected with an N-52 magnet, as the crown side would drag, while the pavilion would not.


or maybe by using a simple loupe...... :wink:

ciao
alberto

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 Post subject: Re: Puzzling Red Garnet?
PostPosted: Wed Jul 13, 2011 12:08 pm 
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Also most garnet topped doublets date back a long time. In a conversation with Gem A shop Alain Clark said the gem-A get most of their GTD from old jewellery from the victorian era. So the age of the parcel doesn't disallow a GTD

Lustre difference between crown and pavilion should be visible with overhead lighting. The joint is not neccessary on the girdle.


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 Post subject: Re: Puzzling Red Garnet?
PostPosted: Wed Jul 13, 2011 12:10 pm 
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Frank wrote:
The joint is not neccessary on the girdle.


yeah, and if it's not and the stone is loose it's quite more easy to spot.

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 Post subject: Re: Puzzling Red Garnet?
PostPosted: Wed Jul 13, 2011 12:17 pm 
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lol looks like we were typing at the same time Alberto...also that our brains were telepathically linked at the time :D


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 Post subject: Re: Puzzling Red Garnet?
PostPosted: Wed Jul 13, 2011 1:14 pm 
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8) :smt025

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 Post subject: Re: Puzzling Red Garnet?
PostPosted: Wed Jul 13, 2011 7:27 pm 
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Hi Alberto
Yes, you are spot on with your remarks re garnet/paste doublet.

The join is obvious (about 2/3 up the crown) once your mind has cottoned on to the possibility. Best to examine stone at low power 10x loupe first then go to higher power.
I get low marks for that. Got carried away with looking at the nice inclusions in the garnet and ignored the few tiny bubbles which were a give away.

Hi Kirk, re magnetism of red paste/garnet doublet
Resting on paper no movement in any orientation
Suspension by string: yes, weak attraction
Flotation on cork: When table facet down no attraction
Lying on pavilion with table facet exposed it shows attraction and cork moves.

Also I have another little red paste/garnet doublet (I knew about this one) which is a 5mm square cut. Table face down on paper it appears inert. Turn it the other way to expose table facet and it will drag.

So all is well. Moral to the story .. always expect a doublet
They must date back to before 1910? or before the production of flame fusion ruby, so would be common in Victorian jewelry. Gemologists who deal in antique jewelry must be having a good laugh when they read this post.
Cheers
Allano


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