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Amethyst or "Grape Agate"?

Posted by Vik Vanrusselt  
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Paul Brandes May 17, 2018 02:27PM
Grape Agate is a marketing name for these kinds of specimens coming out of Indonesia. One of the descriptions (884479) does include the statement "Analysis by Dr. George Rossman at Caltech concluded these should be labeled as amethyst, and not chalcedony or agate." I tend to believe these shouldn't be called Amethyst, but on the other hand, they also should not be called Agate. Maybe a better term for Mindat purposes may be to place these under "Amethystine quartz"? Not true Amethyst, but certainly not Agate either.
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Douglas Schonewald May 17, 2018 03:39PM
If these are macro-crystalline quartz colored purple by the process or irradiation and iron inclusions why would they not be called amethyst? A quartz crystal is still a crystal even if it is very small. I would think that if there is an analysis by a reputable source that their findings should be followed until it is proven they are incorrect or unsubstantiated.
If the crystals are analyzed and found to be colored by a different process then they should be called something else. Perhaps then amethystine would be correct.
Of course, if mindat's definitions are incorrect for either amethyst or amethystine, then all bets are off and the definitions should be updated and/or changed.
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Alfredo Petrov May 17, 2018 03:48PM
I call them "botryoidal amethyst", at least the ones where quartz crystal terminations are visible. (But the crystallinity varies and some of the more cryptocrystalline ones might perhaps be better called "amethystine chalcedony".)



Edited 1 time(s). Last edit at 05/17/2018 03:49PM by Alfredo Petrov.
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Ed Clopton May 17, 2018 05:33PM
This material doesn't really look botryoidal, either, as the term is usually applied in mineralogy--although the aggregates are in fact more "grape-like" than most material conventionally described as being botryoidal. "Oolitic" (fish-egg-like) and "pisolitic" (pea-like) are already taken for textures of limestone and related rocks, and "orbicular" for jasper/chert/chalcedony containing globular structures. How about "spheroidal quartz", either amethystine spheroidal quartz in the case of the purple ones or green spheroidal quartz for the green ones?
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Alfredo Petrov May 17, 2018 05:41PM
The green ones are colored by some clay and/or celadonite inclusions; it isn't really green quartz.
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Gregg Little May 17, 2018 08:10PM
Since some of the samples appear macro-crystalline and others micro-crystalline then classification should be quartz for the former and chalcedony for the latter with any other descriptors (agate, amethystine, etc) assigned to the variety term. As Vik indicates this would keep it mineralogically correct and allow embellishments to run rampant in the variety category. At least most people can see the presence of crystalline surfaces without magnification.
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