CIBJO releases Gemmological Special Report: considers process of separating measurable facts from opinion; See Gemological Articles below.
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 Post subject: Scapolite vs. Amethyst
PostPosted: Sun Jun 18, 2006 12:54 pm 
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On another forum, the topic of seperation between scapolite and amethyst is being discussed.
RI: Scapolite in Uniaxial - (negative)
Amethyst is Uniaxial + (positive)
Polariscope:
Looking for bulls-eye will identify quartz.
Also any evidence of Brazil twinning will indicate amethyst

Can anyone add any other features that will positively seperate the two? :?


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PostPosted: Sun Jun 18, 2006 6:46 pm 
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Off hand I would say birefringence could separate the two. RI may as well, although I think material in the color range of amethyst, those properties may overlap in both stones.

If the stones are included there may be some diagnostic features. Scapolite has perfect cleavage in 2 directions at 90deg. Quartz doesn't.

Amethyst may have zebra striping inclusions that I don't believe would occur in scapolite.

Optic figure is pretty easy to obtain in quartz, so the bulls-eye figure would be a good ID.


Zebra striping in amethyst below:
Image


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PostPosted: Mon Jun 19, 2006 12:28 pm 
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If you had to merely sepearate those two, optical sign is sufficient. If the higher reading varies the most, it's amethyst as the optical sign is calculated as "extra-ordinary ray minus ordinary ray" .. the ordinary ray is always constant in unixial stones.

In Scapolite the higher reading would be fixated (the ordinary ray) while in Amethyst the lower reading is fixated on your refractometer.


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PostPosted: Mon Jun 19, 2006 1:52 pm 
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Yes, I agree that optic sign would be the definitive test for the separation of these two stones. It would definitely be on the list if I were doing a formal colored stone report as proof to it's identity.

If I were just poking around at home and discovered a bulls eye optic figure in the polariscope, I don't think I would even bother with the refractometer.


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PostPosted: Mon Jun 19, 2006 2:13 pm 
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I also understand that this seperation is quite apparent with Visual Optics.

I don't own any purple scapolite. So, I can't play with it. The only time I've ever encountered it was during the final exam at the DGemG.


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PostPosted: Mon Jun 19, 2006 9:24 pm 
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A little more as discussed.


http://www.gia.edu/newsroom/issue/2798/ ... ails.cfm#4


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PostPosted: Tue Jun 20, 2006 12:02 pm 
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Barbra wrote:
I also understand that this seperation is quite apparent with Visual Optics.

I don't own any purple scapolite. So, I can't play with it. The only time I've ever encountered it was during the final exam at the DGemG.


V.O. can be very tricky. I don't have any purple scapolite either but I have some yellow to compare with citrine. Both stones are ovals with the same cut style: modified Portuguese pavilion with square checker crown. I can only 'guesstimate' pavilion main angles.

I don't know whether to believe the books or my own lying eyes. My observations indicate the quartz has a much higher B.D. than the scapolite despite the 0.7 to 1.3 ratio given in the literature. In fact I'd think the scapolite is SR based on my V.O. results.

Maybe an expert like Hodgkinson can make the "Kentucky windage" allowances required by cut variations from the standard round brilliant. I have problems with it. I did find one tier of pavilion facets in the scapolite that gave textbook results but my memory is so bad I'd probably have trouble recalling the need to look for that anomaly in a real-life situation.

As Dr. Hanneman once wrote: "...the critical factor determining the appearance of the images is the angle at which the facets are cut. Since these are never measured, mastery of this technique remains in the realm of an art tempered by experience and desire."

He was writing in 1980, before the explosion of non-traditional cuts. I haven't tried but suspect the technique is useless on concave cuts and perhaps even step-cut stones.

ROM


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PostPosted: Thu Jul 30, 2009 8:16 pm 
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If the RI and BR and SG are overlapping...

Amethyst: Uniaxe positive
color: purple
inclusions: zebra strips
UV long: inert
UV short: inert

Scapolite: Uniaxe negative
color: purple with a hint of grey (it can also be yellow or colorless)
inclusions: thin "ducts" (I mean thin tubes)
UV long: inert
UV short: inert or reddish after 30 seconds of exposure


Last edited by cascaillou on Sat Aug 01, 2009 12:18 pm, edited 2 times in total.

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PostPosted: Thu Jul 30, 2009 11:12 pm 
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I wouldn't rely on the color, nor the UV for identification.
I think determining the optic character with either the refractometer or polaricope is a better idea.


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PostPosted: Fri Jul 31, 2009 5:55 am 
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my mistake, I forgot to write it


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PostPosted: Fri Oct 30, 2009 11:51 am 
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ROM wrote:
Barbra wrote:
I also understand that this separation is quite apparent with Visual Optics.”

ROM responded:
V.O. can be very tricky. I can only 'guesstimate' pavilion main angles.

I don't know whether to believe the books or my own lying eyes. My observations indicate the quartz has a much higher B.D. than the scapolite despite the 0.7 to 1.3 ratio given in the literature. In fact I'd think the scapolite is SR based on my V.O. results.

Maybe an expert like Hodgkinson can make the "Kentucky windage" allowances required by cut variations from the standard round brilliant. I have problems with it. I did find one tier of pavilion facets in the scapolite that gave textbook results ..

As Dr. Hanneman once wrote: "...the critical factor determining the appearance of the images is the angle at which the facets are cut. Since these are never measured, mastery of this technique remains in the realm of an art tempered by experience and desire."
ROM

The Hodgkinson Method originally estimated RI and the determination of RI does indeed require one to consider the pavilion angle or the angles of the facets involved—but that is all.
My contribution to the technique involved the introduction of the concept of the B : D ratio.
When using the B : D ratio to characterize a gem, the angles of the facets are immaterial. You MUST look through more than one set of facets.
This is because, although Dispersion is essentially constant, the appearance of the rainbows is determined by the Birefringence of the stone.
HOWEVER, birefringence varies with the direction one is looking relative to an optic axis.
THEREFORE, one can observe an infinite number of possible B : D ratios.
CONSEQUENTLY, your task is to observe as many sets of rainbows as you believe necessary to allow you to choose the ONE SET showing the maximum separation. Then, determine its B : D ratio. That is the only one that counts.

If you do not have the will to do this, then you should not even attempt to use the method, as you will only end up blaming the method for your own shortcomings.


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PostPosted: Wed Dec 02, 2009 6:25 pm 
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Scapolite vs amethyst. I've been thinking about this again...and I need your participation :-)

for those who have purple scapolite on hand, are these getting redish-orange under chelsea filter or not?

the more people doing this test and reporting results the better, this way the results would be somewhat representative (we would need at least 10 stones of different origins beeing tested).


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PostPosted: Wed Dec 02, 2009 6:45 pm 
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I have purple scapolite, but no chelsea filter... I'll check next time I'm at Barbra's. ;)


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PostPosted: Fri Dec 04, 2009 6:22 pm 
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EDIT: I was at a mineral & gem show today, and tested a few five purple scapolites with my chelsea filter, the bad news is that they all went orange under the filter, just like amethyst would. So my idea that maybe chelsea filter could help in differentiating the two was wrong. That would have been too easy.


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 Post subject: Re: Scapolite vs. Amethyst
PostPosted: Wed Jun 12, 2019 9:07 am 
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Hi all,

I am just seeing this now however I am glad to read this. I actually give a purple scapolite as one of the test stones for ID to gemologists that apply to the lab...

It is a really good test to check the base skills of a gemologist for gem ID. Although not a standard gemological tool/observation, the mid- IR spectrum provides a ready means to separate these two. A tool that is objective and not subject to mis-reading, which can happen with the 'human error' of reading an RI and determining +/- U character.

Best regards to all,
Christopher P. Smith


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