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 Post subject: Biregringent stone in a spinel lot
PostPosted: Tue Apr 06, 2021 6:06 am 
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Hi (gemology newbie here, hello to you all :-) ),

I recently bought some tumbled spinels (origining from Ratnapura), but one of them seems to be something else.

I first noticed when I checked them using a N52 magnet that a 1.55 ct stone of them actually picks up, contrary to what I read about magnetism in spinels.
Apparently spinels generally should only have a low magnetic susceptibility (and the other spinels in the lot aren't even dragged by the magnet), so I checked the stone in the polariscope.

The major part of the stone is not translucent enough to get a reading, but the small purple blob protruding from the rest is clearly birefringent. It also shows a relatively weak pleochroism (switching between shades of purple). If I turn the polarization filters from crossed to parallel while the stone is brightest, its brightness doesn't change, so I am pretty sure this is no anomalous birefringence.

A check with a LW UV light showed no discernible fluorescence (again in contrast to most of the other spinels), maybe due to high iron content?

The spectroscope gives a strong absorption line in the yellow range and a broad absorption band around bluish-green, but due to the size of the stone it is a bit hard to see more details.

I also tested the major red part for hardness and it's clearly around 8 or higher. I do not have a refractometer, so I can't give a refraction index.

Considering all the data, there isn't much left for what it could be.
Wishful thinking says it could be a Taaffeite (with high contents of iron?), but I am sceptical (although the color would match with specimen from the same locality).

However, I can't think of any other explanation for all phenomena (except for an extremely iron-rich purple spinel with extremely strong anomalous double refraction, but Ockham's Razor doesn't really apply there anymore).

Am I missing something or have I actually been lucky?

Best regards!


Attachments:
File comment: View of the stone, with the transparent part on the upper right. Thumbs up...?
photo_2021-04-06_11-54-44.jpg
photo_2021-04-06_11-54-44.jpg [ 29.9 KiB | Viewed 120 times ]
File comment: Same stone, from another view, with the transparent part more visible.
photo_2021-04-06_11-54-48.jpg
photo_2021-04-06_11-54-48.jpg [ 26.88 KiB | Viewed 120 times ]
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 Post subject: Re: Biregringent stone in a spinel lot
PostPosted: Tue Apr 06, 2021 7:13 pm 
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Images are very blurry.

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 Post subject: Re: Biregringent stone in a spinel lot
PostPosted: Tue Apr 06, 2021 8:19 pm 
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And, whenever dealing with red singly refractive stones don't forget about ADR: Anomolous Double Refraction.

Steve, 1bwana1, is good at sorting that out with the polariscope.


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 Post subject: Re: Biregringent stone in a spinel lot
PostPosted: Tue Apr 06, 2021 9:01 pm 
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One instantly thinks of Ruby or Garnet as most likely to get mixed into a parcel of Spinel.

I really would need to examine in person to make any kind of meaningful determination.


Just a couple of minutes with my RAMAN and we would know all.


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 Post subject: Re: Biregringent stone in a spinel lot
PostPosted: Tue Apr 06, 2021 9:37 pm 
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Steve, I was referring to your ability to seperate singly refractive stones displaying ADR with the polariscope.

I, too, have been spoiled with advanced instrumentation.


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 Post subject: Re: Biregringent stone in a spinel lot
PostPosted: Tue Apr 06, 2021 9:39 pm 
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Just as a general note, don't put too much stock in your hardness tests as a beginner. It's easy to get wrong readings, particularly if you don't have a set of styluses (stylii?). I save seen people online get misled by this, in addition to being a frequent problem when TAing intro geology labs.

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 Post subject: Re: Biregringent stone in a spinel lot
PostPosted: Sat Apr 10, 2021 2:10 pm 
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Why don't try S.G.?


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