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 Post subject: Re: General gemmology questions
PostPosted: Thu Jan 28, 2016 2:12 pm 
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Thanks Isi! So the vague shadow edge is due to the surface being somewhat flat and polished but not well enough to get the birefringence?


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 Post subject: Re: General gemmology questions
PostPosted: Thu Jan 28, 2016 7:59 pm 
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Most probably.
This is not only for quartz. On a soft stone, even very flat, if it is scratched, you will be in trouble to read the birefringence, and sometimes the RI. Apatite for instance. Because the surface is uneven, the optical junction doesn't work correctly.

Very often students mistake this case for a OTL stone. Because they don't check with spot lecture (distant vision) that the stone is still in the reading range.


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 Post subject: Re: General gemmology questions
PostPosted: Fri Jan 29, 2016 4:52 pm 
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Isi wrote:
Very often students mistake this case for a OTL stone. Because they don't check with spot lecture (distant vision) that the stone is still in the reading range.

If students were taught to FIRST use the Hodgkinson Method on faceted stones, they could forget all this crap about OTL. :D


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 Post subject: Re: General gemmology questions
PostPosted: Fri Jan 29, 2016 5:33 pm 
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The Hodgkinson method, if I was taught it properly, works if the stone is
- transparent
- not too dark
- not too small
- not too included
- faceted (but only in a certain way)
- ...

an even so, it is very difficult to master for plenty of people.
Otherwise, it would be much more widely used, and you wouldn't have to try to convince us over and over again.

The good ol' refractometer, coupled to a bit of brain and experience, works for all types of stones and all types of people, hence its success.

As Barbra said in another thread, the few OTL stones, once you have determined they are really OTL (thanks to the refractometer), are usually fairly easy to identify with some simple tests available to all. Nothing crappy about it.


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 Post subject: Re: General gemmology questions
PostPosted: Fri Jan 29, 2016 9:52 pm 
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Isi wrote:
The Hodgkinson method, if I was taught it properly, works if the stone is ...

Sorry, but you weren't taught properly. The blind leading the blind. :(


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 Post subject: Re: General gemmology questions
PostPosted: Sat Jan 30, 2016 12:58 am 
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If I may....I was privileged to have Alan Hodgkinson come to my home for a GIA Alumni gathering. He taught us all the principles of visual optics.
We gathered in my dining room, passed around unknowns and Alan (along with Charlotte) walked us, step by step, through precisely what was optically necessary to identify the characteristics of a stone.

I believe the circumstance of this event would indicate that I was taught properly. Can I teach other folks to do what I was taught? Yes.

Can this technique positively identify the unknowns I often encounter? No.
I'll go a step farther. Hell, no!

It is unfair to dismiss alternative, and more definitive methods of gem identification in favor of visual optics.

When someone hands me an unknown, I rely on the most efficient, accurate instrumentation I have in my arsenal.
My reputation depends on it. Identification can not be speculative. There is no room for error.


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 Post subject: Re: General gemmology questions
PostPosted: Sat Jan 30, 2016 3:52 am 
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I am afraid I have been taught by Mr Hodgkinson too, once he came to Paris to explain the method (visual optics, B/D ratio and so on) So I must have been very dumb, just as every other person of this training, who gave up very fast !


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 Post subject: Re: General gemmology questions
PostPosted: Sat Jan 30, 2016 9:53 am 
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What are you using OTL to stand for? From your comments it looks like the RI of the stone is higher than the contact fluid/refractometer scale?


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 Post subject: Re: General gemmology questions
PostPosted: Sat Jan 30, 2016 10:16 am 
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GemmoLogic wrote:
What are you using OTL to stand for? From your comments it looks like the RI of the stone is higher than the contact fluid/refractometer scale?


Yes, "over the limit" of the contact fluid's RI.


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 Post subject: Re: General gemmology questions
PostPosted: Sat Jan 30, 2016 11:15 am 
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Thanks :)


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 Post subject: Re: General gemmology questions
PostPosted: Fri Mar 18, 2016 3:32 pm 
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Hello again,

I have a question about the TIR refractometer. If it works by TIR from the interface between the glass table and the gem, so we don't measure any light that has gone through the gem, how is it possible that it matters what orientation the c-axis is in relation to the facet under test as to the behaviour of the shadow edges?

That may well be a stupid question, but I don't see it explained in any textbooks?!

Thank you :)


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 Post subject: Re: General gemmology questions
PostPosted: Fri Mar 25, 2016 5:45 pm 
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Barbra Voltaire wrote:
Yes, gypsum will fizz as a consequence of the calcium content.
CaSO4·2H2O
This is the reason that karstification* processes take place in gypsum rocks as well.

Fizzing is generally recognized by the rapid generation of carbon dioxide from mineral carbonates in water of an acidic nature. Karstification is a very slow reaction limited by solubility.


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 Post subject: Re: General gemmology questions
PostPosted: Fri Mar 25, 2016 6:10 pm 
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Bill Hanneman wrote:
Barbra Voltaire wrote:
Yes, gypsum will fizz as a consequence of the calcium content.
CaSO4·2H2O
This is the reason that karstification* processes take place in gypsum rocks as well.

Fizzing is generally recognized by the rapid generation of carbon dioxide from mineral carbonates in water of an acidic nature. Karstification is a very slow reaction limited by solubility.

Rainwater is not very acidic so it's a slower process, but the acidity is certainly influential.

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 Post subject: Re: General gemmology questions
PostPosted: Sun Mar 27, 2016 7:02 pm 
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I am sorry, but you missed the point if my last reply.
Fissing has nothing to do with calcium.
Fissing is due to the reaction of a material containing a carbonate reacting with an acid.
This is a matter of chemistry, not gemology


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 Post subject: Re: General gemmology questions
PostPosted: Sun Mar 27, 2016 8:51 pm 
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Heh, I definitely made my reply in too much of a hurry. Normally I think only of limestone in karstification (in which case the solubility of calcite is strongly influenced by the acid-base reaction), but of course it includes gypsum deposits as well, in which case yeah there's no acid-base action going on.

All that to say, no gypsum won't fizz, but it'll dissolve in acid.

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