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 Post subject: Separating Similar Stones
PostPosted: Tue Sep 19, 2006 7:01 am 
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Hi All,
I hope somebody can share their knowledge with me I would love some feedback on the following,

Without using Refractometer or Specific Gravity can someone please tell me how to separate the following gemstones:
demantoid garnet and peridot - i am thinking polariscope
green tourmaline and adalusite - i am thinking dichroscope
amethyst and purple sapphire - hand lens for typical inclusions??
diamond and colourless CZ - spectrascope?
citrine and scapolite - UV Scapolite will go Pink
colourless topaz and colourless zircon - spectrascope?

Thanks....


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PostPosted: Tue Sep 19, 2006 9:25 am 
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Hi Leigh and welcome,

You could seperate almost all with the polariscope (and the addition of a conoscope/retardation filter) except for the Diamond - CZ.

What are you looking for? Fastest method or possible techniques?


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PostPosted: Tue Sep 19, 2006 10:10 am 
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Thanks Doos for your prompt response.

I have not used a retardation filter with a polariscope and only have used conoscope to identify quartz. If retardation filter needed to separate other stones can you suggest some other techniques? Amethyst and Sapphire I could use polariscope and conoscope to find the bullseye unique to quartz. I would like to know the techniques to separate conclusively each pair of stones I listed. Have read Peter Read's Gemmology chapter on conoscope but have not put into practice separating uniaxial and biaxial stones. Is it a tricky techique? UV to separate Citrine and Scapolite or do you think conoscope would be the better way to go? Just not sure which is the best way to go. What about the diamond and CZ? Splash water over the two see which one gets wet?


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PostPosted: Tue Sep 19, 2006 10:30 am 
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The methods you describe could perhaps seperate the gem varieties you listed.
Example.
An amethyst can be seperated from a sapphire with a conoscope or orientation of inclusions (if indeed inclusions are present and you are able to orient the stone in a way where you can get an identifiable optic figure), but what if the mystery stone could be amethyst, sapphire, spinel, purple tourmaline, iolite, tanzanite, garnet ( I saw a lavender grossular last week) or purple scapolite?


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PostPosted: Tue Sep 19, 2006 10:56 am 
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Quote:
I would like to know the techniques to separate conclusively each pair of stones I listed.


Hi Leigh,

Here is one of the problems with that statement. You want conclusive results, but you're limiting yourself to instruments that may not give you conclusive results.

Remember, gem identification is about proving what a gem isn't by testing
until you are left with only one possibility.

By limiting your equipment, you may be limiting your ability to eliminate possibilities.

Certainly some of those pairs of stone will be easy to separate by visual inspection alone, but will it reveal the true identity?

Peridot and zircon would have strong facet doubling so you could separate them from topaz and garnet.

Andalusite would have stronger pleochroism than tourmaline.

There would be different visual characteristics between CZ and diamond.

Inclusions could offer clues for amethyst or sapphire.

UV reaction may occur with scapolite.

In all of those examples, you may be able to separate two similar looking stones, but for conclusive evidence of identity, you'll probably need to use more testing equipment.

Of course, some exceptions could be made for someone who has 20 or 30 years experience working with all these gems. :)


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PostPosted: Tue Sep 19, 2006 11:05 am 
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Thanks JB for your input. I know I said conclusively separate without using refractometer and specific gravity. This is an assignment question I am doing. I was just checking with you guys whether what I thought would be the best tests were, or if I should try other instruments.

You guys are great in replying so fast. Thank you so very much. I have just read Doos article in the polariscope forum and was blown away with his diagrams.

I can't believe I have taken so long in finding this website but am so glad that I have now :D


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PostPosted: Tue Sep 19, 2006 11:13 am 
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Assuming that the stones are faceted, wouldn't the visible double refraction of a peridot be a dead giveaway? That is going on the assumption that your assignment says "you have two green stones, and you know that one is a demantoid and the other is a peridot, how do you tell which is which?" You wouldn't need anything more than a loupe for that.


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PostPosted: Tue Sep 19, 2006 12:07 pm 
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Hi Leigh,

It is not easy to give a definate answer to your seperation question. As this is an assignment we should not provide you with the answers, but as you show your willingness to learn and read, here are some of my suggestions.

When answering such a question/assignment you should atleast do 1 confirmation test. Never trust on one test alone.
And since it is a theory assignment, you can use any instrument you like except the refractometer and the hydrostatic balance/heavy liquids.

Demantoid/Peridot: polariscope - spectroscope - CF (chelsea filter)
Tourmaline/Andalusite: dichroscope - polariscope with conoscope
Amethyst/Sapphire: polariscope with conoscope and retardation plate - inclusions
Diamond/CZ: tilt test - dot test (both described by Read if the stone is a round brilliiant) - UV may be additional - spectroscope not really - 10x loupe
Citrine/Scapolite - polariscope with conoscope and retardation plate - UV
Topaz/Zircon: polariscope with conoscope - spectroscope

You can not seperate Quartz from any other uniaxial stone by trying to find a bulls-eye. There is naturally occuring Quartz that is untwinned. Only if the bulls-eye is present it will be positive, the reverse is not true.

I'm sure others will have much more to add.

p.s.: If you really want to play smarty pants, you could determine the RI with a Brewster Angle Meter or a Table Spectrometer. And of course there is the Visual Optics technique.


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PostPosted: Wed Sep 20, 2006 3:16 am 
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Thanks for your reply Doos. I really did think about the question myself before posting it here on the board. I just wanted confirmation that I was going in the right directions with my thoughts or if not a better test for each one. Just one question about separating CZ and Diamond. In your reply you said not really to using a spectroscope. Using a spectroscope wouldn't I see a "cape line" at 415.3 nm in a diamond and what is termed "picket fence" ie lines scattered across spectrum in the CZ? Your thoughts on this would be appreciated.

Thanks


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PostPosted: Wed Sep 20, 2006 10:07 am 
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It is my understanding that the "cape Line" in diamonds is extremely difficult to see unless the diamond has a rather pronounced body color: M+.
May I ask what school you are doing this project for?


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PostPosted: Wed Sep 20, 2006 10:07 am 
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Hi Leigh,

Not all diamonds belong to the cape series and that line might be very hard to see. I never heared about "picket fence" lines in CZ .. could you or anyone fill me in on that?
I never use a spectroscope on diamonds, so someone else should step in here.

Come to think of it, the polariscope might help seperating well proportioned brilliant cut diamonds and CZ .. if you uncross the filters little to no light should pass through the diamond (table down) due to TIR.
As I don't have a nice collection of diamonds to play with, maybe someone could test this?
If my thoughts are correct, all the pairs can be seperated with the polariscope alone (of course additional tests should be done).

Out of curiousity Leigh, for which institution are you doing this assignment?


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PostPosted: Wed Sep 20, 2006 10:08 am 
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Lol Barbra .. we posted the exact same comment/question in the same minute.


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PostPosted: Wed Sep 20, 2006 11:23 pm 
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I am doing my course through the Victorian Division of Gemmological Association of Australia.

Thanks for your help guys. It is really interesting hearing your feedback.


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PostPosted: Thu Sep 21, 2006 8:42 am 
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Leigh, you have one of the best gemmology teachers in the world right there at the school .. Annie. My personal gemmology heroine.
She can provide you with much better answers than I, trust me.


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PostPosted: Thu Sep 21, 2006 10:45 am 
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Welcome Leigh, its great to have you here.

I just saw this post, Please forgive me everybody. !! I haven't been online much lately. Very hectic schedule - been flat out.

You must be in Gem1 year.. I demonstrate on tuesday nights; If I can assist in any way, please don't hesitate to come and see me and we can discuss it in detail, if you like; are you practicing past end of year exam paper questions ?- what assignment is this; the exams are on in 3 weeks. .. Of course it never hurts to repeat and revise and link your theory into practice. Repeation is the key. You have a great year ahead of you still which you will and should enjoy and be proud of.

The very first thing you should always pick up is your handlens to see or examine internal and external features, inclusions are of importance to the gemmo world, easiest way of separating stones from another and diagnostics. Then of course you can move onto other instruments and each of your instruments will give you a clue to conclude which will come in handy for next year.


To put it simply, for now:
Demantoid - high dispersion, lucky for horsestails /Peridot - lilypads or disc like healed fractures.

Tourmaline - doubling of back facets due to high birefringence, separate Andulasites from its trichroic appearance alone to the naked eye.

Amethyst - colour and possibly zebra stripes to /Sapphire - straight colour bands or zoning..

Diamond - sharp facet edges /CZ: facets edges may look rounded

Citrine - dichroscope - observation of differential selective absorption, as there will be no dichorism, due to heat treatment /Scapolite of course UV is best; Citrine is inert.

Topaz - colour blue, incipient cleavages, any.. /Zircon: doubling of back facets

Then of course, you have all the other equipments as our friends have suggested above.

cheers,
Annie


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