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Gemstone's surface pattern: any ideas?

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  1. Nov 2, 2017 #1
    emerald.png
    ruby.png
    2 SEM images of gemstones: emerald and ruby (Electron Miscroscope with Secondary Electron detector, (which "takes picture" of surface layer (~0.5 um) both the elemental composition and relief)).

    Dendrites - it is clear (not me, but in general), what creates other patterns?
    * trace elements? - the stones are very clean (<1000 ppm) and well processed
    * relief? - OK and next question: why such relief?
    * electrostatics? - definitely not. Electrostatics in dielectrics looks like distortion at the top of ruby photo.
    * anything else?

    I would be very grateful for any ideas! Or any links.
     
  2. jcsd
  3. Nov 10, 2017 #2
    Fascinating pictures. In the upper one, how about microcractures related to the underlying crystal structure? Although they look initially random, there seem to be certain preferred orientations. I shall follow the thread with interest.
     
  4. Nov 10, 2017 #3
    I strongly recommend you go over to https://www.gemologyonline.com/Forum/phpBB2/index.php

    I'm going to post a link here over there and see what happens.(also, they have a sub-forum specifically for gemological photos of gem inclusions, i.e., any imperfections inside a gem one can see)

    There are best-in-class gemologists there, including some with heavy interest in physical chemistry, geochemistry, optics, and instrumentation. I think they would be able to actually answer some of your questions. Or tell you "it is unknown"... with some degree of authority and certainty.

    I mean... they LIKE real scientific publications and some are real (professional) scientists that do gems for fun or a living. (disclaimer- I was active there 5+ years back, the "good" folks may not be around anymore)

    From the little I know about this stuff, I'll note that natural gems are formed under non-ideal conditions. Crystal strain, partial fractures, partial healing of fractures (by partial melting), crystallographic dislocations, and many more are all well-known to sceince-oriented gemologists. Then, there are human- caused issues like subsurface strain caused by plastic deformation during polishing... fracture-filling... very high temp treatments in varying redox conditions and so on.

    The bottom image looks kinda like partially healed fractures ("fingerprint" inclusions); the top... dunno.

    I have seen other SEM images of polished stones, but none looked anything like these. It is possible even that the images are in essence artifacts of the technique, making interpretation of the data ... problematic.
     
    Last edited: Nov 10, 2017
  5. Nov 10, 2017 #4

    Tom.G

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    The Ruby inclusions sure look like contaminant crystal growth on a solid substrate. If you have been in snow country in the Winter you will see very similiar patterns on the inside of single-glazed windows. The moisture in the indoor air condenses on the cold glass and grows ice crystals in these patterns.
     
  6. Nov 13, 2017 at 4:40 PM #5
    General Q for OP-
    Are those natural gems or synthetic? Note that either type may be synthetic or treated- in particular, natural ruby-colored corundum is frequently treated under conditions that are pretty severe- hot enough to cause partial melting of the corundum, fracture filling with leaded glass, colors modified (even color "created") by various redox conditions at very high (~1200C not uncommon), and color treatments resulting in significant amounts of added beryllium migrating several mm into the AlO2 crystal.

    In fact, there was a big, big stink 5 or 10 years back about high temperature glass-filling. Macy's in particular was selling pretty-looking rubies with having so much glass filling that removal of the glass transformed a nice looking stone into several smaller, ugly chunks- yes, they were literally held together by the glass.



    Now the top... emerald is synthetically manufactured by various methods as well; I now recall some surfaces having macroscopic patterns like shown on the surface.

    Natural emeralds are almost always quite obviously flawed and fractured because they form in water-rich environments (synthetics are grown in essentially an autoclave; high temp + H2O). They are almost always treated by application of oils/resins that fill-in surface imperfections, even significant fractures where they reach the surface- the improvement can be quite remarkable. Oiling is a traditional treatment and pretty much all retail emeralds have been "oiled". That's why emeralds should never be steam cleaned. Resins (epoxies) are less accepted in the trade because they can be hard to spot and can mislead buyers when no oil is apparent.
     
  7. Nov 14, 2017 at 12:53 PM #6
    From the gemology forum:
    1:
    I would guess the emerald surface is mostly partially polished scratches left over from cutting, but that's kind of the easy way out on that.

    The ruby dendrites are really strange. I'd guess it would have to be some (lovely) surface contamination--the orientation is just too perfect. You wouldn't really expect anything like that inherent in the structure since they crystallize relatively slowly and have plenty of time at relatively high temperatures for inclusions to take on more thermodynamically favorable configurations with a better surface area to volume ratio.


    2:
    Hi,
    To be more objective with SEM images in high (10'000 - 100'000x) magnification, more info about SEM imaging condition needed: detector type, vacuum mode, acceleration voltage... very often, that kind of images is generated by surface charging, the best way to verify could be EDX mapping - BSE detector is less sensitive to element distribution than EDX.
    Peters
     
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