CIBJO releases Gemmological Special Report: considers process of separating measurable facts from opinion; See Gemological Articles below.
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PostPosted: Fri Jul 27, 2007 10:03 am 
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Hi,

When we agree on the definition, things may get complicated.
Is rhodolite really idiochromatic?

If iron alone would be the cause of color, then why does it have the colour of those rhodondendron flowers I have in my backyard and not that of a synthezised pyrope-almandine without chromium added?

At first I agreed with you MoDo, but if we are splitting hairs (as I do) .. no gemstone is idiochromatic, nor allochromatic.
It is all interaction.

Time for a new defintion I think.


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PostPosted: Fri Jul 27, 2007 10:40 am 
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Rhodolite garnet is a species of garnet ... if you take away the iron, you take away the color that makes it rhodolite. Hmm ... that would make it a pyrope garnet :smt017 . Okay ... I may have to give you that one as well :lol: . This is confusing 8) . Perhaps the confusion comes from the differences presented by the garnet group that aren't present in other species and varieties :? ??

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PostPosted: Fri Jul 27, 2007 10:51 am 
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I don't see the definition of idiochromatic as an issue. Perhaps more important is how we choose to classify our gems.


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PostPosted: Fri Jul 27, 2007 11:06 am 
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> insert Jeopardy music here <


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PostPosted: Fri Jul 27, 2007 2:56 pm 
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Hi all,

I'd agree with Modo...If the colour is due to an essential part of the chemical composition of the gem then the gem is termed idiochromatic. The most obvious of these in the gem world is probably malachite which is green due to copper.

I also like Annie's means to remember how to differentiate which is which between allochromatic and idiochromatic...idiochromatic stones are those idiots that are just full of themselves

Be well all

Frank


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PostPosted: Fri Jul 27, 2007 3:36 pm 
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"Sorry Frank, we can't give you that, you forgot to buzz in"


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PostPosted: Fri Jul 27, 2007 3:45 pm 
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"Who are three people who have never been in my kitchen?"

Mega-points to whoever can identify that Jeopardy reference... and no cheating by using search engines... I'll know. ;)


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PostPosted: Fri Jul 27, 2007 4:26 pm 
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JB, Buzz buzz

I'm sure these all American jokes are hilarious for those who 'get them'

Who IS the geezer with his hankie in his breast pocket (do people still do that)

Probably some merry kin celebrity that no one else has ever heard of :roll:

saying that I haven't had a TV connected for ten years or so so I probably wouldn't recognise anyone from home either

Ah well I'll just keep quiet so no one realises what a boring old fart I'm become (ing)


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PostPosted: Fri Jul 27, 2007 4:46 pm 
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Sorry Frank. It's hard to find a one size fits all joke anymore.
If you have a minute, I'll tell you what you've missed in the last ten years of being television free.

I'm just killing time until the Quiz answer is revealed. We're starting to travel in circles. :wink:


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PostPosted: Fri Jul 27, 2007 5:53 pm 
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Jeez Frank, perish the thought! Alex Trebec is a Canuck, you silly Scotsman! :lol:


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PostPosted: Fri Jul 27, 2007 7:12 pm 
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africanuck wrote:
Jeez Frank, perish the thought! Alex Trebec is a Canuck, you silly Scotsman! :lol:


He is?! Well that certainly explains THAT!


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PostPosted: Fri Jul 27, 2007 7:50 pm 
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Quote:
Idiochromatic - a (mineral) deriving a characteristic color from its capacity to absorb certain light rays.


Not being a gemologist or a scientist, I am finding this definition confusing as it is referring to absorbing light rays, not reflecting light rays, which if I remember correctly that would account for the color that one sees. Is this just a backward way of saying that its characteristic color is from light reflected? :smt120


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PostPosted: Fri Jul 27, 2007 8:11 pm 
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deriving a characteristic color from its capacity to absorb certain light rays.


What does that mean? What minerals do not have the capacity? :idea:
Colorless minerals/allochromatic. Colored minerals/ idiochromatic.


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PostPosted: Fri Jul 27, 2007 8:45 pm 
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BTW: I know you enjoy torturing us Bill, but, if you don't reply soon, we're going to start making crap up, if we haven't already. :lol:


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PostPosted: Fri Jul 27, 2007 10:52 pm 
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OK, here is my view.

I agree with Barbra, idiochromatic is a perfectly good adjective and the problem is “how we choose to classify gems.” I also agree with Doos (all of them are idiochromatic gemstones), however I’m not certain he felt comfortable with his answer—probably as a result of what he was taught.

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, (e.g., The Garnet, Chrysolite, or Feldspars Groups) are handled, as well as the treatment of color varieties (e.g., beryl and quartz).

The “present apparently unbreakable” rule of mineralogical nomenclature is that where a solid solution solution exists (consider Pyrope - Almandite) 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 it must be termed an allochromatic mineral. However, it can’t be a “Rhodolite” unless it has iron so iron is an essential element and that makes it an idiochromatic gemstone.

If Peridot were a “newly discovered material," it would be mineralogically classified as a color variety of the mineral forsterite which contained about 10 mole% fayalite, and would be considered an allochromatic mineral. However, recognition of peridot as a gemstone preceded the “science” of mineralogy and it got named first. Mineralogy had to accommodate that fact. To do this, a new “species” called chrysolite or olivine, which includes the necessarily undefined region between between forsterite and fayalite, was created.

However, when it came to the Feldspar Group, things became a bit more complicated and the mineralogical solution for the plagioclase feldspars (albite-anorthite) was to divide this binary system into five different regions, calling each a species and precisely defining the chemical composition of each. This had the effect of placing the dividing line between Andesine and Laboradorite precisely at the midpoint.

As long as feldspars were recognized as allochromatic minerals, there was little interest in them as gemstones by anyone, as was the fact one needed a good chemical analysis to separate the two species. However, when it came to color varieties, like sunstone and red andesine, gemologists took notice.

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. (Red andesine is different in that andesine is the name of an allochromatic mineral—not a color variety. It is a material in need of a good commercial name.)


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