New Mineral Named After GIA’s John Koivula
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PostPosted: Fri Sep 01, 2006 9:09 pm 
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Brad -- What JB described is pretty much straight from the GIA's Graduate Gemologist curriculum. False double refraction" may well be a better term from an optics standpoint, but you're likely to only hear about ADR from anyone with GIA training.

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PostPosted: Fri Sep 01, 2006 9:57 pm 
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Hi brad,

You are obviously a very bright fellow, but crystal optics, to the extent that you describe it is way beyond the ordinary gemologists training.

A good gemologist will will be able to tell you if a gem is singly or doubly refractive without intensive theory.

Gemological training isn't science specific, that's why most courses can be completed in a fairly short amount of time.

To break down every specific theory that rubs up against gemology would mean a lifetime of study of geology, chemistry, physics, optics and on and on.

From the practical side of gemology (the part that pays the bucks) it's basically, identifying, buying and selling and appraising gems and jewelry.

If I was younger and was more scientifically inclined, I would have honed in on one of these sciences and sought employment in the R&D department of some petroleum giant or some government funded University research project where the pay and benefits are better than most gemological positions.

Anyway, this can be interesting stuff for some people, but most of it falls beyond what I think I can ever practically apply. Of course, I'm not 18 anymore and don't have 50 years left to sort it all out.

It would cut into my hedonistic pursuits. :)


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PostPosted: Fri Sep 01, 2006 10:19 pm 
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Quote:
Brad wrote:
If possible, it would be good to change your term to 'False double refraction'.

Hi Brad :P
Actually, the first time I was introduced to the terms "anamolous double refration" and "anamolous interference" was in Optical Mineralogy, as a geology student. I believe they were adopted into gemological jargon from their mineralogical roots.


Last edited by Barbra Voltaire, FGG on Fri Sep 22, 2006 1:07 pm, edited 1 time in total.

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PostPosted: Sat Sep 02, 2006 5:00 am 
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Hi All,

Currently the term ADR is being used by everyone but current thought in the recent Gem-A courses is that a better description would be anomalous extinction since it is an extinction effect which is seen and not actually double refraction.

Frank


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PostPosted: Sat Sep 02, 2006 7:29 am 
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O.K. As I guessed, ADR is a firmly rooted term that we can't do anything about. Thank you for explaining this: I have never done a gemmology course so this kind of terminology is new to me.
Just for background information, I too am no chicken ( 60), and gemmology/crystal optics is my recently-adopted hedonisitic pursuit with retirement looming.

In 40 years of teaching, I have found that students seldom have difficulty in absorbing ideas, but confusing terminology really does cause them trouble. As a 17-year old biologist, I remember my problems with antimycin, actinomycin and actomyosin. I wonder how many patients have been killed by doctors who misheard hypertension for hypotension?

At present, throughout science and engineering, field-specific acronyms like ADR are a major block to communication, and we greybeards should do our best to promote plain english. Which reminds me of an argument I have been having with the norwegian gemmologist Halvorsen about his 'Usambara Effect' (colour change dependent on thickness). He has published this year in J. Gemmology on this remarkable (and remarkably common) type of colour change, which is not in the textbooks.
Brad


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PostPosted: Sat Sep 02, 2006 10:30 am 
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Concerning ADR,

Maybe the least confusing and most specific term should be, "Strain Extinction."


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PostPosted: Sat Sep 02, 2006 5:24 pm 
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Quote:
Which reminds me of an argument I have been having with the norwegian gemmologist Halvorsen about his 'Usambara Effect' (colour change dependent on thickness).


Maybe you should invite the norwegian gent to continue the argument with you over here, that sounds interesting.

One of our members mentioned the Usambara effect in an earlier thread
http://gemologyonline.com/Forum/phpBB2/viewtopic.php?t=112&highlight=usambara


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 Post subject: Re: New address for Brad Amos's website
PostPosted: Sun Apr 28, 2019 9:16 pm 
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Peter Torraca wrote:
O.k., I got one very good reply from the faceter's digest. To be honest, much of this is over my head (as with Doos above), but I'm glad for the challenge.

The replies below seem to overlap the Doos explaination above, but there is some new stuff as well -- and a link to some incredible (and huge) images.


Quote:
Re: Polariscope and the c-axis
Posted by: "Dr William B. Amos" bradscopegems
Date: Fri Aug 25, 2006 7:07 pm (PDT)

Dear Peter,
I would not recommend the use of a polariscope in the
case of topaz.
With some materials, such as quartz, corundum and tourmaline, it
works fine: you just put the rough into the polariscope and rotate
it. It goes dark in four positions and the c axis is parallel to the
polariser axis in two of those positions and perpendicular to it in
the other two. The problem reduces to a choice of two positions for
the axis. If you have no clues in the crystal form, you just turn
the chunk all ways until you find a direction of view where spinning
it around the line of view leaves it dark (or at least constant in
brightness) all the time. You are now looking along the c axis,
which in these three minerals corresponds to looking along the long
axis of the hexagonal prism. These minerals are called uniaxial:
they have only one optical axis and it corresponds to the
crystallographic c axis. Topaz, however is biaxial: it has two
optical axes and you cannot discover where they are by the simple
rotation tests. Moreover, they do not have any simple relation to
the crystallographic axes and the angle between the two varies quite
a lot from one topaz specimen to another. Vargas (p68) does not
explain any of this. Having experimented with topaz crystals of
known crystal facet morphology I promise you that you will drive
yourself crazy trying to do it. I finally persuaded Frank Norman to
make a little sphere of topaz so that I could demonstrate just how
complicated biaxial crystals are when viewed in the polariscope. If
you want to get into this, you can download some articles I wrote:
http://homepage.ntlworld.com/w.amos2/Brad%20Amos's%20Website.


(See especially "List of PDF files" then "photographs of topaz sphere..." low bandwidth users beware...)

Quote:

Posted by: "Dr William B. Amos" bradscopegems
Subject: Re: Polariscope and the c-axis
Date: August 30, 2006 3:16:31 AM EST

Dear Peter,
You are welcome to post my reply on the gemmology
online forum.

Those who have studied crystallography will say that it is possible to
do it. This depends on having some way of viewing the so-called
conoscopic pattern between crossed polarisers. There are three ways of
doing this. One is to make a sphere, like the one Frank Norman made
for me. This is hardly practical if you have an irregular piece of
rough and just want to do quick check. The second is to hold a small
spherical bead of glass against the sample and look through both the
bead and the sample between crossed polarisers.. This is often
recommended in gemmology texts. The other is the geologist's way,
which is to cut a section of the gem, polish it on both sides, put it
in a polarising microscope and add an extra lens called a Bertrand
lens, which allows you to see the back focal plane of the objective.
With topaz, all these methods give a curious pattern with two
'eyes' (melatopes). If you can orient the specimen so that you have a
dark line running right across it parallel to either the polariser or
analyser axes and then turn it, keeping the line in view, you will see
the melatopes on the line. The conoscopic pattern is a kind of
angular map, and it tells you the angular position of the optical
axes, which run through the centre of the melatopes. In the centre of
the acute angle between the two optical axes, the topaz will look dark
(zero degrees in my pictures of Frank's sphere). At this position, you
are looking along the c axis.
It is quite tricky to find this position with even a smooth
semi-polished topaz pebble, because the angle between the melatopes is
so large and you are lucky to see even a single one of them. That is
why I don't recommend the method for beginners. However, I do
recommend trying this on a sheet of mica or even a sheet of 'Mylar'
plastic film. The melatopes are worth seeing, though not for people of
a nervous disposition, since they look like devil's eyes! A really
easy way to see them is to make a sandwich of a piece of mica between
two crossed polarising films, hold it close toyour eye and gaze at a
large white area such as a wall or a sheet of paper. The melatopes
will loom up at you.
The final twist is that in some specimens of topaz the
angle that is normally acute apparently becomes greater than 90
degrees, so the 'acute bisectrix' then becomes perpendicular to the c
axis. I have not seen this, but it is yet another difficulty to
contend with.
Post this bit too, if you wish.

Brad

Apologies to all. Time has moved on.The above website address is defunct. My personal web address is now
http://www.brad-amos.webplus.net.
The articles on birefringence are still there. I am no longer the UltraTec rep for the UK: it is Steve Sweetman, who can be contacted at http://www.bespokegems.uk and is a champion faceter.


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