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GeneralWhat's the deal with etched minerals?
29th Aug 2012 09:32 UTCJohn M Stolz Expert
I have a general question about why etched minerals like beryls and tourmalines are so desireable. I've seen a few--mostly in pictures--and I don't understand why they seem to be preferred over non-etched specimens.
John
29th Aug 2012 13:04 UTCMaggie Wilson Expert
29th Aug 2012 14:36 UTCRobert Simonoff
Bob
29th Aug 2012 14:47 UTCMario Pauwels
Like Maggy said, the "etched faces" of a rough gem crystal have almost always a high luster and show strong reflections, while most "non etched" rough gem crystals not necessarely have that high luster and missing the typical strong "window" reflections on their surface to. Also the typical appealling and repeating "window patern" is something that is highly appreciated on "etched " gem crystals.
Best regards,
Mario Pauwels
29th Aug 2012 15:34 UTCOwen Lewis
But, to address John's point directly, if the plane(s) of a termination cannot (even indistinctly) be seen on a crystal as presented, then to say that there is such a termination (or broken end or contact area) is simply wishful thinking. One simply does not know what may once have been there.
29th Aug 2012 16:18 UTCAlex Homenuke 🌟 Expert
29th Aug 2012 23:31 UTCJohn M Stolz Expert
Another mystery for me that's associated with the subject at hand is how some crystals can show frosting of preferential faces. Owen's pictures kind of show this preferential etching of faces, but the specimen I have--a pakistani aqua--has frosting only of the faces that aren't orthogonal to one of the axes and absolutely no frosting on the remaining faces. I can't visualize a process that would do that, and the internet is of limited use for any kind of plain-english explanation.
@Alex: what does MSH mean?
Regards, John
29th Aug 2012 23:52 UTCStephanie Martin
Mont Saint-Hilaire, Rouville RCM, Montérégie, Québec, Canada
regards,
stephanie :-)
30th Aug 2012 02:56 UTCOwen Lewis
I have a strong impression that the 'outgrowths' are simply that and do not result from etching away of the rest of the surface to leave the 'outgrowths' standing proud. In the case of the almost perfectly flat face, I have never seen any etching effect that could do that. In the case of the pits, yes, etching is a possibility - but why on only the one of the three faces? Given the general present of 'outgrowth' features, is it not at least plausible that the pits might be caused not by etching by some very local effect that arrested/retards the crystal growth to create the pits as the rest of the crystal surface around then grew at a faster rate? A sort of negative growth?
Like others, I feel that there are far more questions surrounding this growth/etching phenomenon than there are sufficient answers to. It would indeed be nice to learn more.
30th Aug 2012 03:10 UTCAnonymous User
As far as the etching solution, I have heard that solutions rich in flourine ion would do it. Aren't these commonly what help to form the pegmatite gem minerals in the first place?
30th Aug 2012 15:55 UTCKelly Nash 🌟 Expert
30th Aug 2012 17:07 UTCRon Layton
30th Aug 2012 18:37 UTCJohn M Stolz Expert
@Kelly--thanks for that--I never considered anisotropy! My bad!
That's quite a mouthful Steph--no wonder you abbreviate it!
John
31st Aug 2012 00:51 UTCRyan L. Bowling
Sharp crystals on matrix are the most desired, I would say in any species, while single crystals mostly fall into a second tier.
Single crystals, either etched or not, have to have special properties for me to like them. Most important in my mind is color, the perfect hue can make any crystal exceptional. Luster is equally important. Get the two together, and one usually has an exceptional gem crystal.
Etched beryl crystals many times have exceptional growth patterns, almost like finding oneself lost in a crystal maze.
I myself, have three etched gem crystals, all Brazilian. I love them all, great color/growth patterns, and luster. I would take a fine colored etched Brazilian aqua, over a pale colored Pakistan aqua on matrix, any day of the week.
All the best,
Ryan
31st Aug 2012 15:58 UTCJohn M Stolz Expert
I agree with you on the importance of color and luster--that's kind of what prompted my question: I saw an exquisite picture of an etched aquamarine here http://www.mindat.org/mesg-6-269667.html posted by Jason Barrett and I got to wondering if the etching didn't actually enhance the color/luster over what it would present as if unetched... and yet, there's the question of form in addition to color and texture--and I can't help but question the aesthetics of an etched crystal v. an unetched one, all things equal.
I need to spend some time looking at the real things instead of pictures--often I have trouble understanding what the photo is showing.
John
31st Aug 2012 17:40 UTCRonald J. Pellar Expert
I have judged many competition cases and I generally discount the quality score for specimens lacking any morphology, including etched xtals. The less morphology present the more the discount.
To me, beautifully colored, severely etched "xtals" are strictly gem material.
31st Aug 2012 19:54 UTCJohn M Stolz Expert
31st Aug 2012 20:03 UTCAlfredo Petrov Manager
31st Aug 2012 23:43 UTCWilliam C. van Laer Expert
Since some minerals like tourmaline seem to do this often (plus beryl, topaz, and many others) there is a clear tendency for this to occur. In the case of tourmaline, where different colors withing thew same crystal represent differing stabilities, the earlier phases are less stable and tend to "etch" first, hence the irregularity of the example that Dr. White gives. It is NOT due to the deepth of immersion in the pocket fluid, but the differing stabilities of the two phases of tourmaline growth.
Often what is referred to as "etched" crystals are just growth--reverse growth phases repeated, as the line between stability and instability is crossed again and again (a good example of this is the spessartine garnets found recently in Brazil, along with those found elsewhere in pegmatite pockets like the Little Three Mine in California).
It is clear that many etched crystals are less that aesthetic, but there are those that break that rule!
Chris
31st Aug 2012 23:59 UTCOwen Lewis
1st Sep 2012 01:08 UTCRalph S Bottrill 🌟 Manager
15th May 2015 17:16 UTCRoy
although the thread is not a new one, I hope someone can help.
Here a picture of topaz:
http://www.meelis-bluetopaz.com/about-topaz.html
(bottom right)
Could this be an etched topaz crystal?
Or are there naturally occuring topaz crystals with such crystal faces?
Kind regards,
Roy
15th May 2015 17:33 UTCRob Woodside 🌟 Manager
15th May 2015 17:57 UTCRoy
I just found an article about etching of topaz by use of potassium hydroxide or a mixture of potassium
bisulfate and powdered fluorite (The Etching Figures of Topaz, Am. Miner., Vol. 6, No. 4)
But this was done only for a short time to study symmetry features.
I think one could expect the same strange morphology after prolonged periods of etching (?)
Roy
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