October 3-7—JOSHUA TREE, CALIFORNIA: Annual show; Sportsmans Club of Joshua Tree
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 Post subject: Practical Preparation
PostPosted: Fri Nov 06, 2009 5:46 am 
After what seems like ages of putting it off (turns out I only skipped the biannual exam twice) I have decided to finally do the FGA Practical in January. Theory/Foundation are long done-and-dusted - although I'm not sure when they void your marks if you repeatedly do not sit the other exams (three years?).

I did the practical once before, with zero preparation, just to get a 'feel' for it. Turns out this was a wise move, as it was pretty crazy the first time.

Anyway, what exactly should I concentrate on learning to make sure I put the FGA away?

- Knowing the crystal system for anything that might potentially come up seems like a obvious move. Think I have that down already. Obviously this leads to easy iso/anisotropic, uni/biaxial etc. determinations with polatiscopes etc. However, this information is also in the Blue Book, but that takes up valuable time.

- Refractometers seems to be the 'big one'. A whole section devoted to getting birefringences and a spot RI reading is a great ID tool. I just bought my first refractometer yesterday, so hopefully that will be second-nature by January (the other time I did the practical exam was the first and only time I ever touched a refractometer, lol).

- To what extent will they test inclusions? Phenakite in, say, synthetic emerald and 2/3-phase inclusions in natural emerald (etc.)? I can't remember how pivotal microscope use was in this particular exam, but there were microscopes set up. Do you lose ID credit for saying 'synthetic' for 'natural', and visa-versa?

- At the FGA practical revision days what exactly is done? Are they worth while? 200 pounds seems a lot for a single day.

- Can we definitely bring all our own equipment to the exam if it is in the allowed equipment list (i.e. polariscopes, refractometers etc.). What if the equipment is more advanced than the 'norm'?

Cheers,

Simon


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 Post subject: Re: Practical Preparation
PostPosted: Fri Nov 06, 2009 8:25 am 
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Kyriakin wrote:
After what seems like ages of putting it off (turns out I only skipped the biannual exam twice) I have decided to finally do the FGA Practical in January. Theory/Foundation are long done-and-dusted - although I'm not sure when they void your marks if you repeatedly do not sit the other exams (three years?).


Ask 'm... info@gem-a.com
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I did the practical once before, with zero preparation, just to get a 'feel' for it. Turns out this was a wise move, as it was pretty crazy the first time.

Anyway, what exactly should I concentrate on learning to make sure I put the FGA away?


Fast and accurate stone ID. Speed should be your main objective. 8 minutes per stone plus writing down your observations isn't much. Become very familiar with your gear.
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- Knowing the crystal system for anything that might potentially come up seems like a obvious move. Think I have that down already. Obviously this leads to easy iso/anisotropic, uni/biaxial etc. determinations with polatiscopes etc. However, this information is also in the Blue Book, but that takes up valuable time.


The use of the practical guide is allowed ----> take full advantage of it. Know that book inside out so you don't loose time searching for the appropriate pages. When you find the stone you are ID-ing is a sapphire open the book at the sapphire characteristics page and list which ones are present in the stone, this way you don't forget to list obvious properties.

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- Refractometers seems to be the 'big one'. A whole section devoted to getting birefringences and a spot RI reading is a great ID tool. I just bought my first refractometer yesterday, so hopefully that will be second-nature by January (the other time I did the practical exam was the first and only time I ever touched a refractometer, lol).


Practise, practise, practise... That tool gives you so much info. SR/DR, optic sign & character and of course RI and birefringence. You are not allowed to take SG measurements so this tool is the only 'exact' measurement you can take. The backbone of your ID...
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- To what extent will they test inclusions? Phenakite in, say, synthetic emerald and 2/3-phase inclusions in natural emerald (etc.)? I can't remember how pivotal microscope use was in this particular exam, but there were microscopes set up. Do you lose ID credit for saying 'synthetic' for 'natural', and visa-versa?


Of course you loose points when you mis-ID a natural stone as a synthetic or vice versa... Inclusions are mighty important, try to look into as many stones as possible in oncoming months and get yourself close to Gubelin & Koivula's photo atlases, Maybe you can access them at the GIT?
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- At the FGA practical revision days what exactly is done? Are they worth while? 200 pounds seems a lot for a single day.


That will depend on who you meet as your instructor and how experienced you already are. My session wasn't all that exciting and I learned very little I didn't know yet...
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- Can we definitely bring all our own equipment to the exam if it is in the allowed equipment list (i.e. polariscopes, refractometers etc.). What if the equipment is more advanced than the 'norm'?


I brought my own tools, if you are given the opportunity why not use the stuff you are familiar with... It depends on what you have, my stuff was superior to the tools that were present at the Dutch exam centre. If the exam centre you will attend has good stuff while you have Chinese knock-offs I would use theirs...

Good luck mate, remember: speed... Practise till you can ID a stone within 4-5 minutes... If you get stuck during the exam start on a new stone immediately move on, don't loose precious time pondering on what it may be... You can always return to it.


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PostPosted: Fri Nov 06, 2009 11:41 pm 
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I admit it was long ago that I took the practical (1983), but...when I took it there were five Site -ID stones that were rough crystals. My advice is for you to go to your local mineral shop and buy rough samples of: apitite, topaz, herkimer diamond (quartz), doubly terminated sapphire, flat ruby crystal, tourmaline, spinel, and whatever else you can afford (if you don't have them already). Handling these items is worth more than a thousand words in a text book and will immidiatly help you out if they show up. Tiny samples are as good as large ones. And yes, I had to take five R.I.'s, but no ID's of faceted stones. things may obviously be different today. Good preperation is better than good luck. Regards, Richard.

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 Post subject: Re: Practical Preparation
PostPosted: Sun Nov 08, 2009 9:27 am 
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- To what extent will they test inclusions? Phenakite in, say, synthetic emerald and 2/3-phase inclusions in natural emerald (etc.)? I can't remember how pivotal microscope use was in this particular exam, but there were microscopes set up. Do you lose ID credit for saying 'synthetic' for 'natural', and visa-versa?

Quote:
Of course you loose points when you mis-ID a natural stone as a synthetic or vice versa... Inclusions are mighty important, try to look into as many stones as possible in oncoming months and get yourself close to Gubelin & Koivula's photo atlases, Maybe you can access them at the GIT?


I was really asking in regards to the "Three strikes and you're out" policy of Section D. I remember that failing to ID four stones in this section would give you an automatic fail. Was wondering if a mix-up in natural/synthetic, however with the correct species, would count as one of your 'three failures'.

Also, with respect to refractometers:

- Why does the instruction manual say to close the lid and turn the polariser? I seem to have got the right result with nearly every stone so far with the lid up (so turning the stone is quick) and ignoring the polariser on the lid.
The exeption to this was a Burmese peridot which gave this weird set of rainbow-like patterns on the scale, rather than the expected two shadow edges. Closing the lid was the cure in this case.

- Does the maxima of the upper-RI line and the minima of the lower-RI line occur at the same orientation? That seemed to be roughly the case with the last stone I tested.
If rotating the stone while viewing the scale found this point of maximum birefringence, would this be enough 'workings' for the markers, or would they want a full set of readings for a 180-degree rotation?

- Similarly, is the point at which the stone seems to have nearly no birefringence (low-max high-min) going to occur at one set orientation? Would this be perpendicular to the maximum-birefringence point mentioned before?

- Final question. Do some peridots ever almost appear uniaxial? I had one stone where the lower-RI line literally didn't move in all orientations. The upper line did, however move enough to compensate and still give a 0.035ish birefringence result.


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PostPosted: Sun Nov 08, 2009 12:24 pm 
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One determines the birefringence with the aid of the polarizer. It is not function of just turning the stone and recording RI.
Closing the lid removes ambient light and the readings are crisper....especially when turning the polarizer.

I close the lid with every reading and always use the polarizer. I often find that the birefringence can be equally as diagnostic as the RI.

Alan Hodkinson does some tricks with an open refractometer and overhead light shining above the stone; the technique will be described in the book he has been working on for the last several years.


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PostPosted: Sun Nov 08, 2009 2:43 pm 
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- Final question. Do some peridots ever almost appear uniaxial? I had one stone where the lower-RI line literally didn't move in all orientations. The upper line did, however move enough to compensate and still give a 0.035ish birefringence result.


Hi Simon,

Peridot is right on the dividing line between uniaxial positive and uniaxial negative. In fact either reading is possible. If you have a situation where only one line is moving then you may find that you are on one of the two optic axes. This would cause one of the lines to 'freeze'. I'd suggest trying for a reading on another facet.

In my office I tend to leave the lid open on the refractometer which is positioned so that not much ambient lighting gets into it. Elsewhere I'd tend to close the lid. The polariser is used to cut out one of the two shadow lines so that accurate measurements can be made with a single shadow line. After reading the first set of measurements the polariser is rotated 90 degrees and the first shadow edge disappears leaving the second one to be rotated and measured. This makes accurate birefringence readings on biaxial stones much easier. Like Barbra I find that an accurate bifrefringence reading is as important for positive ID than just the RI value.


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PostPosted: Sun Nov 08, 2009 10:23 pm 
I am taking birefringences too though. While viewing both lines at the same time, with the lid open, my worst peridot birefringence value (from eleven stones) was 0.034 (where 0.036 is the given value in most sources).

I am a bad writer (can't write joined up, and medical issues cause me to cramp), so will accept a small compromise to save time - as long as the birefringence values are close. The ridiculously unrealistic time constraints of the exam force my hand on that a bit (some girls in the last exams missed up to 6 stones in Section D).

I have to say, though, that in all eleven stones the peridot was defininitely +ve. I haven't yet seen a stone where the range of RI values is similar in both lines, let alone a stone where the lower-RI line has the greatest range.

Unfortunitely, all my non-cab stones are garnets, spinel and peridots, so I don't have much else to measure birefringences with. Been trying to practice the distance view measurements with the cabs though.


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PostPosted: Sun Nov 08, 2009 10:49 pm 
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Frank wrote:
After reading the first set of measurements the polariser is rotated 90 degrees and the first shadow edge disappears leaving the second one to be rotated and measured.


So do I understand correctly that you put the polarizer in place, take measurements in all positions, THEN rotate the polarizer and re-rotate the stone? I've been rotating the polarizer at each position, but if my understanding of your method is correct, I can see how that might be considerably quicker.


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PostPosted: Mon Nov 09, 2009 3:28 pm 
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So do I understand correctly that you put the polarizer in place, take measurements in all positions, THEN rotate the polarizer and re-rotate the stone?


Yes that is the way I do it. When the stone is first placed on the refractometerI first look at it without the polariser to determine if isotropic or not. If two shadow edges are seen then I'll put the polariser on the eyepiece and slowly rotate until one of the lines disappear. Then rotate the stone through all positions. The turn the polariser 90 degrees and the rotate the stone again and take a second set of readings.

It is of course possible to do it without a polariser and for Gem-A exams it would be better to do it more quickly than this but for sure results this is a good technique and with practice can be done pretty quickly.


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PostPosted: Tue Nov 10, 2009 9:44 pm 
Ages ago bought an 'unknown' stone (shopkeepers words) that I assumed to be a tourmaline. Kinda bluish-green and oval, with a small windowed table.

Just put it through the refractometer, and it does indeed have a RI of 1.63, a DR of 0.021 and is negative.

However, the high-RI line does move a little bit (maybe 0.005). Can this sometimes happen in tourmaline? Or is my Chinese knock-off refractometer letting me down? Or am I just crap?


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