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 Post subject: Re: Yellow stone
PostPosted: Thu Mar 07, 2013 9:16 am 
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What surprises me is the difference in RI if you measure one facet (1.61) or the other (1.69). Makes no sense to me. It is really a big difference, 0.08, nearly 0.1. There is no reason for that.

It would be easier for you to pick among the right stones if the RI and consequently the birefringence were more accurate.


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 Post subject: Re: Yellow stone
PostPosted: Thu Mar 07, 2013 11:22 am 
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There is more to reporting on a refractive index than supplying one number.
The stone should be rotated, viewed with a polarizing plate, noting highs and lows.
This will enable you to determine optic character and proper birefringence.
http://www.gemologyproject.com/wiki/ind ... ractometer

We are questioning your results because there is no gem with the SG and RI you are reporting.


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 Post subject: Re: Yellow stone
PostPosted: Thu Mar 07, 2013 4:11 pm 
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BJBecker wrote:
There is some flux with the RI based on the facet angle used, if I use the pavilion main angle of 50ish degrees then RI is 1.61, if I use the pavilion star at 42, then i get closer to 1.69. I am sure though the RI is at least 1.6x.

Tools: Hanneman refractometer,
Your problem is with the star facet. Forget about that, as it is too difficult to measure acccuractly.
Your birefringencen is probably OK.


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 Post subject: Re: Yellow stone
PostPosted: Fri Mar 08, 2013 4:44 am 
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Dr. Bill-
Your right. I matched the projected image to the worng facet. I figured this out by doing a ray trace (good old paper and protractor). When I use the pavilion main (49 degrees) I get 1.578 with a bifring of .005. I will post my ray trace later if anyone is interested to double check my math.


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 Post subject: Re: Yellow stone
PostPosted: Fri Mar 08, 2013 10:56 am 
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I'm interested BJ.
Your new RI makes a profound difference.


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 Post subject: Re: Yellow stone
PostPosted: Tue Mar 12, 2013 11:56 am 
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Not that this really helps with the ID since I'm still between Citrine and Beryl (with Citrine being the most likely) just like I was in the original post, but the picture is becoming clearer (or muddier). Anyway, back to deciding if the 42 deg pav star or the 49 deg pav main is projecting the image I used on the refractometer. The image is the 4th of 5 images and is the clearest (5 facets in the pinapple) so with make that image. Here is the math related to the pencil and paper ray trace I ran (couldn't get it to scan well). I used RI of 2.17 as this is to simply determin with facet gives me the further image.

For 42 deg
<I = 41 Deg
crunched through RI = sin<I/sin<R
I get <R = sin<sup>1-</sup>(sin41/2.17)
this give me <R = 18 deg

ploted through to the table I get:
<I = 22 deg
<R = sin<sup>1-</sup>(sin18/1)
<R = 22 deg
on my Hannamen Refreactometer (5 5/8 in tall) that gives me a distance of:
m = tan22deg*5.625 =m = 2.27 in

For 49 deg
<I 48 deg
<R = sin<sup>1-</sup>(sin 48/2.17)
<R = 20 deg

plotted to the table I get:
<I 29 deg
<R = sin<sup>1-</sup>(sin29/1)
<R = 29

wich would give a distance of:
m = tan 29*5.625 =m= 3.11

so the 49 deg facet gives the further out image that being said here is all the accumulated raw data for this stone.

Hannamen refratometer distance is 5 5/8 in (from table of gem to ruler)

so at polarity 1 I get the following distances:
3 3/16, 3 /11/16, 3 12/16, 3 14/16, 3 8/16, 3 12/16, 3 10/16, 2 31/32, 3, 3 1/32, 3, 3 3/32, 3 6/32
Ave Measure = 3 163/416
< 31.1 deg =tan(sup>1-</sup> (3 163/416 / 5 5/8)
using Dr. Hannamen charts 31.1 deg exit angle on a 49 deg pav = 1.538

polarity 2 is:
4 8/16, 4 5/16, 4 5/16, 3 12/16, 3 13/16, 3 8/16, 3 8/16, 3 1/32, 3, 2 30/32, 3 1/32, 3 1/32, 3 4/32
ave Measure = 3 219/416
< 32.1 deg =tan<sup>1-</sup>(3 219/416 / 5 5/8)
so 32.1 deg exit on a 49 deg pav = 1.556

mean RI = 1.547 for this data set

Based of the 49 deg pav main angle, and this cumulative data set my best guess is citrine.

Thanks for helping me refine my technique.


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 Post subject: Re: Yellow stone
PostPosted: Tue Mar 12, 2013 2:33 pm 
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 Post subject: Re: Yellow stone
PostPosted: Tue Mar 12, 2013 3:48 pm 
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Sorry, BJBecker, but do you realize that someone with a standard refractometer (and the knowledge to use it) could separate citrine from beryl in less than 10 seconds?


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 Post subject: Re: Yellow stone
PostPosted: Thu Mar 14, 2013 8:04 am 
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BJBecker wrote:
  Post subject:  Re: Yellow stone  
Not that this really helps with the ID since I'm still between Citrine and Beryl (with Citrine being the most likely) just like I was in the original post, but the picture is becoming clearer (or muddier). Anyway, back to deciding if the 42 deg pav star or the 49 deg pav main is projecting the image I used on the refractometer. The image is the 4th of 5 images and is the clearest (5 facets in the pinapple) so with make that image. Here is the math related to the pencil and paper ray trace I ran (couldn't get it to scan well). I used RI of 2.17 as this is to simply determin with facet gives me the further image.

Thanks for helping me refine my technique.

Keep up the good work, BJBecker. You now know the laws of optics work and how to use them for identification. Now, you can use your refractometer for measuring the RI, Birefringence, and Dispersion for all those OTL stones which cannot be done with a critical angle refractometer.


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 Post subject: Re: Yellow stone
PostPosted: Thu Mar 14, 2013 11:53 am 
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Barbra-
While using a "standard" refractometer would have made this easier, I wouldn't have learned as much. My 12yr son can stick a rock in a box and push a button to get results as well (no offence meant) that doesn't mean that he really understands what is going on in the stone. Being an amature hobbiest I picked the Dr. Hannamen's refractometer because it gives me the best bang for my buck. With a little bit of work I can get a decent ball park RI on any transparnt stone and I don't have to be limited to a set RI limit. Also, I'm learning to use all the gemological tools avalible to me and not just relying on SG and RI to make a decissin. If you will remember in my original post I had already narrowed the stone down to citrine or beryl (and was asking about their magnetic response), but the help that everyone gave me in dealing with the RI issue helped me to positively ID it, as well as refine my techneque and learn to better understand my equipment and optics better. The whole point of challenging myself to ID these stones is to learn the process and principles behind gemomolgy, not always to make a 100% positive professional ID. Again thanks for all your help with this stone, I've learned a lot and look forward to contributing more mystory stones in the future. Thanks


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 Post subject: Re: Yellow stone
PostPosted: Thu Mar 14, 2013 12:49 pm 
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BJBecker wrote:
While using a "standard" refractometer would have made this easier, I wouldn't have learned as much.
Have you identified the stone yet? I'm not clear on the purpose of this exercise. What, exactly, are you learning how to do?
BJBecker wrote:
My 12yr son can stick a rock in a box and push a button to get results as well (no offence meant) that doesn't mean that he really understands what is going on in the stone.

What box is this? I'd consider buying one. I'm not aware of the existence of such a box. :smt102

If you are referring to sophisticated spectrographic instrumentation enabling someone to identify elements which can not be otherwise determined with standard gemological instruments, I must point out these units (Raman, LIBS, UV-Vis-IR, XRF,. SEM, SIMS, etc.) are currently used by PhDs like John Emmett, George Rossman, Ken Scarrett, Skip Williams, Brendan Lars, Christopher Smith, Emmanuel Fritsch, Henri Hanni, Ulrich Henn for various gemological purposes, not the least of which are identifying possible treatments gems have undergone before appearing in the marketplace. To infer that these industry icons lack an understanding of what is "going on with a stone" because analysis involves pushing a button....... is a little naive. :)

BJBecker wrote:
Being an amature hobbiest I picked the Dr. Hannamen's refractometer because it gives me the best bang for my buck. With a little bit of work I can get a decent ball park RI on any transparnt stone and I don't have to be limited to a set RI limit. Also, I'm learning to use all the gemological tools avalible to me and not just relying on SG and RI to make a decissin.

Correct me if I have overlooked something, but it doesn't appear the instrument has produced the data which would enable you to positively identify the unknown. So how has this tool been useful?
Using a polariscope and refractometer you could have made the determination last week in 2 minutes.
Bill Hanneman wrote:
Keep up the good work, BJBecker. You now know the laws of optics work and how to use them for identification.
Clearly if that were true, the stone would have been identified by now. :roll:


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 Post subject: Re: Yellow stone
PostPosted: Thu Mar 14, 2013 10:08 pm 
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Barbra Voltaire wrote:
Correct me if I have overlooked something, but it doesn't appear the instrument has produced the data which would enable you to positively identify the unknown. So how has this tool been useful?

The question was it citrine or beryl. He determined it was citrine by using this tool. He wrote that based on the 49 deg pav main angle, and this cumulative data set my best guess is citrine. You apparently don't understand how this refractometer works.


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 Post subject: Re: Yellow stone
PostPosted: Thu Mar 14, 2013 11:17 pm 
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Quote:
You apparently don't understand how this refractometer works.

You are quite right. :|

With all due respect, how long do you think one would last as a salaried gemologist/appraiser if it took 1/2 a day to compile enough information to make a "best guess" on a stone's identity.

Heaven forbid if this mystery stone was a yellow scapolite.


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 Post subject: Re: Yellow stone
PostPosted: Fri Mar 15, 2013 6:09 am 
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Maybe the wisest and most efficient would be to stick to the standard refactometer for all stones with an RI under 1.78, and to use this other tool (which I do not know either, by the way - but I am very eager to learn !) to help for the ID of stones OTL ?

I really don't think that the standard refactometer is a box with a button to push to give you the right answer. Quite the contrary in fact. Of course many people use it without really understanding the optic principle, and it works somehow, but when you try to think and to understand how and why it works, you can very well find out much about what goes on in the stone. In my opinion this refractometer is just a marvel of precision and ingeniosity, and it gives you a lot for its value (even if you find it expensive at first !). Especially if you are an amateur hobbyist with no intention or prospect to ever work in a lab with real costy machines.

I don't know how you can do without a critical angle refractometer, if you want to work seriously, but of course this doesn't exclude to try as many other tools as possible. Anything that can help is welcome.


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 Post subject: Re: Yellow stone
PostPosted: Fri Mar 15, 2013 11:23 am 
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Barbra-
I really don't mean to ruffle soo many feathers, but unlike like you and the other professional gemologists here I can spend 1/2 a day or a weekend examining the stone and only get a "best guess". I don't have any money riding on my identification, Justin and his business has no money riding on the identification, and I've never claimed to be anything other than a hobbiest at best. With this set up, I can turn a light on and have a nice picture of the prismatic affect the stone is having on the light. I can see the wide dispersion displayed by a CZ, and by simply rotating my polarized plate I can get a feel for the birefringence. This refractometer set up is very similar to the Hodgskins method of holding the table up to you eye and looking through the stone to make some basic judgments. If I remember correctly you held a party for this, and even taught you son how to make the technique work?

As for a "black box" as I understand the GIA refractometer works like all basic portable refractometers. So correct me if I'm way off base, but you apply a drop of fluid to you prism to achieve a thin layer. Gently place the stone on the prism, close the lid. Using a monoChrom light source (sodium lamp ideally) you take a reading for where the edge of the shadow line falls. The stone is then rotated several times on the same polarity and then the procedure is repeated at the cross pol setting. The difference in readings let you determing optic sign, birefringence and dispersion. That procedure can be taught to anyone who can follow directions, and you really don't need to understand the underling optical properties to make it work. Though it can help. Without a doubt there are a lot of PH. Ds. working on gems and IDing them and their compsit elements using X-rays and mass specs, and electron microscopes etc but those instraments are not needed to simply ID a stone, and I don't recall any electorn or x-ray microscopes at my local jewelers gemologists lab.

As to the point of my exercise, it is simple: take a set of unknown stones and ID them to the best of my ability with the equipment that I have and can afford. What am I learing form this:
-great refresher in basic optics and geometry and trig
-this also applies to teaching myself how to facet
-using said images to help my 12yr old son in undertanding the geometry that he is starting to learn in math, and re-enforce some basic math skills he is learning but struggling with
-learning more effective problem solving techniques, and helping him to truely understand what is meant by the emperical method of: hypothosize, test hypothysis and gather data, evaluate data, and then re-evaluate the hypothysis in the light of the data to see if it stands or needs to be changed
-I'm also learing and gaining confidence in my ability to ID stones in "field" conditions with "hand-held" tools so that I'm less likely to be as badly cheated when I buy beads or cabs for the wire work jewelry that I make. I'm not saying everyone is a cheat, but I don't have to be a sucker. I'm sure when you go to a large bead/gem show like Tucson they like you to pull out you refractometer at their both and take a reading.
-and considering that I'm dong this on my own, with no foramal training, have no access to a set of reference stones for comparison or hi end professional equipment, or GG, FGA or PH. D teachers to ask, and that this stone is about 18th stone that I've worked with I think I'm doing rather well.

Again I really don't mean to ruffle so many feathers, or tick people off. I'm just trying to learn how to make an educated guess about and ID some gemstones that I was able to get free access to. Again sorry to ruffle feathers and upset the "status quo". And like I said in my original post I had it down to citrine or beryl, and am positive that it is citrine. Also the Chelsea filter reaction was inert in case you are wondering.


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