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 Post subject: Questions for Tim
PostPosted: Tue May 13, 2014 1:05 am 
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Spessartite-grossular... heck, why not throw some almandine in there as well... I tried to blur the lines between the end-members and introduce the concept of isomorphous replacement. The 'need to name' issue is silly in my eyes.

So you believe the “need to name” is, silly.
Me thinks that thou art “pulling my chain,” but I will bite. :lol: :lol:

I believe the history of gemology (and everything else) in the world shows that a name is essential in order to communicate. While hundreds of years ago garnets were “all” red. Today they come in all colors, they all have names, and the more precise the name the better the communication.

Are you now suggesting one goes back to just calling them just garnets?

Are you really a die hard mineralogist who has no use for gemological jargon?
How about “idiochromatic” and “isochromatic”?
Are they just simply equivalent forms of isometric replacements and need not be differentiated?

How is a newbee to learn about gemology if they are taught names are irrelevant or silly?

I believe, gemology is based on names. No one ever sells corundum if they can call it a ruby. Given a choice, few would buy a spessartine-almandine garnet over a spessartine-grossular garnet. I take it you disagree. :cry:


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 Post subject: Re: Questions for Tim
PostPosted: Tue May 13, 2014 4:49 am 
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Hi dr. Bill,

Sorry I got you worked up, that wasn't my objective at all. Like I said, I just wanted Mahina to learn about isomorphic series (link). You are creating your own discussion here by putting all this in my mouth but heck, since you flattered me by putting my name in the title, I'll bite too :P . Since you take one of my words and make a lot from it, I would like to reserve that right too. Please excuse any expansions of topic.

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Me thinks that thou art “pulling my chain,” but I will bite. :lol: :lol:


Nope, I wasn't. Mahina had clearly found the name game, I was just making sure that some mineralogy made it in there too (by linking to an abstract of your work)

Quote:
I believe the history of gemology (and everything else) in the world shows that a name is essential in order to communicate. While hundreds of years ago garnets were “all” red. Today they come in all colors, they all have names, and the more precise the name the better the communication.


And the better the sales. No argument there. Since 'gemologists' survive on the task of justifying prices set by people who are just looking to make their product more unique they better learn their trade names too.

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So you believe the “need to name” is, silly.


Si senor. Just from a common sense point of view. Do you call a scotch & soda a soda just because the majority of the contents is soda?

And while we are at it... trade names are even more fun. Coincidentally I was teaching a class about garnet names last Friday. I started with plotting the mineralogical end members on the whiteboard and threw two of your pyramids into the class to show them the correlation between end member concentration and optical properties. Then I started adding trade names for the various colors. The reactions from students, still unaffected by 'the history of gemology', was quite clear: " :shock: this is nuts!". Hence my 'silly'. It's a great way of introducing them to the 'anything goes' mentality of what people call the 'gem trade' though.

Quote:
Are you now suggesting one goes back to just calling them just garnets?


Nope.

Quote:
Are you really a die hard mineralogist who has no use for gemological jargon?


I'm just a rockhoud who dislikes unnecessary complication. I'm not a retail gemologist if that helps form the picture. I teach the goldsmiths of tomorrow how to buy, work and sell gems and do introduce them to 'gemological jargon' (the lawless stuff) but only after I introduce them to the mineralogical jargon (the stuff that does make sense).

Quote:
How about “idiochromatic” and “isochromatic”?
Are they just simply equivalent forms of isometric replacements and need not be differentiated?


That might be one way of approaching it when in a philosophical mood... :-k Aren't all idiochromatic gems part of isomorphous series that have allochromatic members? (this is an honest question, I can't think of any that aren't at the moment). Isn't a peridot really a forsterite doped with iron?

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How is a newbee to learn about gemology if they are taught names are irrelevant or silly?


They aren't (when you define gemology as the art of selling gemstones).

Quote:
I believe, gemology is based on names. No one ever sells corundum if they can call it a ruby. Given a choice, few would buy a spessartine-almandine garnet over a spessartine-grossular garnet. I take it you disagree. :cry:


You correctly connect 'gemology' with 'selling'. Every customer will prefer 'rhodolite' over 'pyrope-almandine garnet'. I don't disagree with the statement that during a sales talk appealing names are more effective. We weren't selling Mahina a gemstone though. Nor are (trade) names useful to material scientists who are figuring out how to develop detection methods for treatments, synthetics and origin determination - the stuff that 'the trade' really needs in order to keep the trust of 'the buyers'. These guys are the 'real heroes of gemology' :wink: . (now I am pulling your chain)

Ow yeah, in regard to the guy getting a SR 1,81 reading on his TIR refractometer...

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Quote:
No, I don't think so. He was just using his refractometer in the way it was designed and using its full range, not the way students are now tought.


How exactly must this guy have been using his refractometer?


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 Post subject: Re: Questions for Tim
PostPosted: Tue May 13, 2014 9:28 am 
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Well I do wish I had read this post before responding to the above one.

Too late alas, however that is how it works for me. Garnets are a complex series so confusion does reign, at least in the books I have come across.

I would like to have DR. Hannemann's Garnet book but it does seems difficult to come by.

Seeing as I've dug myself into a rather deep hole, best I say no more. I doubt my simple naming system will catch on.

Regards,

Sean.


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 Post subject: Re: Questions for Tim
PostPosted: Tue May 13, 2014 2:48 pm 
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Quote:
How about “idiochromatic” and “isochromatic”?
Are they just simply equivalent forms of isometric replacements and need not be differentiated?


That might be one way of approaching it when in a philosophical mood... :-k Aren't all idiochromatic gems part of isomorphous series that have allochromatic members? (this is an honest question, I can't think of any that aren't at the moment). Isn't a peridot really a forsterite doped with iron?

I invite you to chew on the following? :D :D

My dictionary gives the following definitions:

Chromaticity -noun- (optics) The quality of a color as determined by its dominant wavelength and its purity.

Idiochromatic -adjective- a (mineral) deriving a characteristic color from its capacity to absorb certain light rays.

Allochromatic -adjective- a (mineral) having no color in itself but bearing colored impurities.

My question is, “Which of the following gemstones are idiochromatic?”
Rhodolite
Tsavorite
Peridot
Ruby
Sapphire
Rhodochrosite

-----------------------

One must remember, the word “idiochromatic” is an adjective. It has no real meaning until one knows what noun it is modifying. Words mean something. Therein lies the problem.

The original question was not, “Which of the following minerals are idiochromatic?” but rather “Which of the following gemstones are idiochromatic?”

There is a significant difference between gemological nomenclature and mineralogical nomenclature—especially in the way groups composed of solid solutions are handled, (e.g., Garnet or Olivine).

Rhodolite
The “present unbreakable” rule of mineralogical nomenclature is that where a solid solution solution exists (consider Pyrope - Almandine) the dividing line is drawn at 50 mole%. From this, it follows that the material which was originally described as the color variety “Rhodolite” is two thirds pyrope garnet. Therefore a rhodolite garnet must be considered an allochromatic mineral. On the other hand, it can’t be a “Rhodolite” unless it contains a iron to provide that characteristic color by absorbing certain light rays., Since iron is now an essential element, that makes Rhodolite an idiochromatic gemstone.

Tsavorite
By the same reasoning, Tsavorite is an idiochromatic gemstone, but still an allochromatic mineral of the species Grossular.

Peridot
Peridot is no longer a recognized mineral. However, recognition of peridot as a gemstone preceded the “science” of mineralogy and it got named first and early mineralogical nomenclature had to accommodate that “grandfathering.” To do this, a species called chrysolite or olivine, which includes the necessarily undefined region between between forsterite and fayalite, was created. But time changes things.
Today, mineralogically, peridot is recognized as a color variety of the mineral forsterite. However, today, mineralogical nomenclature committees recognize they have no control over the naming of color varieties and are content to let the marketplace be the judge and simply acknowledge the existence of those names which “stand the test of time.” Contrary to what was accepted in the past, peridot must be considered an allochromatic mineral., but it is an idiochromatic gemstone.

Ruby and Sapphire
Ruby is an color variety of the allochromatic mineral corundum. However since it cannot be a ruby unless it contains chromium to provide its characteristic color, it is an idiochromatic gemstone.
Sapphire is also a variety of the allochromatic mineral corundum. However, it is not a color variety because it exhibits no characteristic color. Consequently, it is an allochromatic gemstone.

Rhodochrosite
Rhodochrosite is a classic example of both an idiochromatic mineral and an idiochromatic gemstone. However, it is not a color variety.

Color varieties of allochromatic minerals are idiochromatic gemstones.

While I have no problems with the mineralogical classifications of allochromatic and idiochromatic minerals, I do not believe gemologists have any good reason to adopt it, for the reason it does not consider the existence of color varieties—the heart and soul of the gemstone trade.

I believe gemology would be far better off if students were simply taught to recognize that color varieties of allochromatic minerals are idiochromatic gemstones.

Once a student knows emeralds, aquamarines, rubies, and iolite are idiochromatic gemstones, such terms as “red emerald, and colorless iolite will be recognized as the oxymorons they are and will not even consider discussing them.

I think Random House's definition of the adjective “idiochromatic,” which incorporates the prefix idio- signifying “proper to one” or “peculiar” and derived from the Greek idios meaning “one, personal, private, separate, or distinct” is excellent. However, note it makes no reference to any degree, kind, or cause for this capacity to affect light, only that it do so.

I have no problems with the mineralogical definitions: “Idiochromatic - a (mineral) deriving a characteristic color from its capacity to absorb certain light rays.” and “Allochromatic - a (mineral) having no color in itself but bearing colored impurities.
I will happily leave them to the mineralogists who prefer to deal with pure species of their ideal chemical formula (To them, everything else is an impurity). Most gemstones don’t fit that criterion as those “impurities” are essential components of gems.

For gemology, I would propose the following:

Idiochromatic - a (gemstone) deriving a characteristic color from its capacity to absorb certain light rays.

There is really no need for the term Allochromatic, as the prefix “ ” is Greek and simply means “other.” Therefore a gemstone is either idiochromatic or not. The operative word in all this discussion is the word “characteristic.”


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 Post subject: Re: Questions for Tim
PostPosted: Tue May 13, 2014 4:14 pm 
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Cool post! Thinking outside of the box :D

Quote:
For gemology, I would propose the following:

Idiochromatic - a (gemstone) deriving a characteristic color from its capacity to absorb certain light rays.


Quote:
I believe gemology would be far better off if students were simply taught to recognize that color varieties of allochromatic minerals are idiochromatic gemstones.

Once a student knows emeralds, aquamarines, rubies, and iolite are idiochromatic gemstones, such terms as “red emerald, and colorless iolite will be recognized as the oxymorons they are and will not even consider discussing them.


Interesting point but I worry a little about exploding brains in my classroom... I'll save this for the advanced course and file it under the additional, optional course 'philosophical gemology'. :mrgreen:

In the meantime I rather use mineralogy as a firm basis from which we occasionally venture into the snakepit of trade names. I dislike having to to provide meaning to trade names, no matter how long they have been used. Every time gemologists try to cook up some chemical laws to define a trade name (think Paraiba) things go south pretty quick.

Also, in isomorphous series things are a bit different than in allochromatic minerals ( 8) minerals - I'm learning!). The concentration of iron/almandine in pyrope is significantly higher than the few ppm of chromium we need to color a bit of corundum red.

And what about gems colored by crystal defects? Add a few hundred degrees or, in some cases, lay 'm in the sun for an x amount of time and they'd go from idio- to allochromatic gemstones when using your definition.

Nope, I'm sticking to the mineralogical definitions and will never use the word 'gemstone' behind 'idiochromatic' again. Promised.


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 Post subject: Re: Questions for Tim
PostPosted: Sat Jun 21, 2014 10:13 am 
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hehehe...some great banter provided by Tim and Dr. H....love the info!

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MrAmethystguy ~ Some jokes just fluorite over my head!


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