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 Post subject: Explanation of internal strain
PostPosted: Fri Jun 08, 2012 12:21 pm 
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Could someone please explain to me what is going on when a gem has strong internal strain? What's happening in the crystal lattice that causes "strain"? Extra atoms? I feel like I can't find a definition of what internal strain actually is.

I remember reading some posts here about "fake" doubling of facet junctions that can occur in singly refractive stones due to strain, too. Where can I look to find out more about that?

Thank you!


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 Post subject: Re: Explanation of internal strain
PostPosted: Fri Jun 08, 2012 2:18 pm 
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Harper wrote:
Could someone please explain to me what is going on when a gem has strong internal strain? What's happening in the crystal lattice that causes "strain"? Extra atoms? I feel like I can't find a definition of what internal strain actually is.

I remember reading some posts here about "fake" doubling of facet junctions that can occur in singly refractive stones due to strain, too. Where can I look to find out more about that?

Thank you!

internal strain is a stress set up inside a gemstone results in structure irregularities (deformation of a crystal, when its shape is twisted out tend to stretch, to compress to shear or change its dimensions) such as twining or distortion
strained structure in crystal's lattice exhibits a peculiar patchy extinction under crossed polars of polariscope, if you mean those parallel lines which occurs on the surface of a crystal it is twinning.

as for doubling by strain its a property of transparent mineral or gemstone by which birefringence induced by the application of mechanical stress or rapidly cooled substance

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 Post subject: Re: Explanation of internal strain
PostPosted: Fri Jun 08, 2012 3:29 pm 
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Thanks for answering!

roshanravan wrote:
internal strain is a stress set up inside a gemstone results in structure irregularities...


But what's the source of stress that results in the distortions? Edit: Like, what would cause a garnet (for example) to have internal strain as opposed to one that did not?

roshanravan wrote:
as for doubling by strain its a property of transparent mineral or gemstone by which birefringence induced by the application of mechanical stress or rapidly cooled substance.


So the distortions could cause a light ray to split into two and show two images like an anisotropic stone...? What's an example of mechanical stress?


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 Post subject: Re: Explanation of internal strain
PostPosted: Sat Jun 09, 2012 3:22 am 
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Harper wrote:
But what's the source of stress that results in the distortions? Edit: Like, what would cause a garnet (for example) to have internal strain as opposed to one that did not?

well through Earth phases and changing temperature & pressure environment also other factors like Pole shifts and so on can cause the stress.
this link also might help you understand causes of stress in earth: http://www.ehow.com/info_8468366_three- ... crust.html
Harper wrote:
So the distortions could cause a light ray to split into two and show two images like an anisotropic stone...?

The distortion can change RI or birefringence, i have not experienced doubling by stress
maybe it is not notable that much in SR stones....
Harper wrote:
What's an example of mechanical stress?

for mechanical stress read here: http://www.britannica.com/EBchecked/top ... properties
i did use geology terms which i've learned in University maybe it was not good answer to your question, mechanical stress can also done by positive and negative electric charges like piezoelectric effect of quartz, but i guess that was not your first question

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 Post subject: Re: Explanation of internal strain
PostPosted: Sat Jun 09, 2012 8:37 am 
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Quote:
roshanravan wrote:
Harper wrote:
But what's the source of stress that results in the distortions? Edit: Like, what would cause a garnet (for example) to have internal strain as opposed to one that did not?

well through Earth phases and changing temperature & pressure environment also other factors like Pole shifts and so on can cause the stress.
this link also might help you understand causes of stress in earth: http://www.ehow.com/info_8468366_three- ... crust.html


Your link makes me think you are thinking way to big Roshan. Internal strain just occurs when the (micro)environment the stone grew in was anything other than perfect. As you can imagine, this is often the case. Stuff cools a tidbit too quick, pressures change, availability of elements changes etc etc. Take a look at the dotted examples in this link for a heap of strain examples.


Quote:
Harper wrote:
So the distortions could cause a light ray to split into two and show two images like an anisotropic stone...?

The distortion can change RI or birefringence, i have not experienced doubling by stress
maybe it is not notable that much in SR stones....


Roshan is right there, you will not get any DR on your refractometer. ADR only becomes clear under a polariscope. Why is light getting through your second filter at her polariscope? Harper, it's not that the isotropic stone causes a doubling of rays, it's changing their polarization plane. The vibration plane of the polarized light from the first filter is 'bent' towards the transmission plane of the second filter. So.. no doubling, but a change of plane of polarization is causing you to see light (ADR).


Quote:
Harper wrote:
What's an example of mechanical stress?

for mechanical stress read here: http://www.britannica.com/EBchecked/top ... properties
i did use geology terms which i've learned in University maybe it was not good answer to your question, mechanical stress can also done by positive and negative electric charges like piezoelectric effect of quartz, but i guess that was not your first question


Check link above for some clear examples (dotted ones).


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 Post subject: Re: Explanation of internal strain
PostPosted: Sat Jun 09, 2012 9:43 am 
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Tim wrote:
Your link makes me think you are thinking way to big Roshan.....

yep as i've replied i went from gemology to geology because i thought Harper's question was referring more in geology... :idea:

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 Post subject: Re: Explanation of internal strain
PostPosted: Sat Jun 09, 2012 10:01 am 
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I found the post that spurred my second question about a doubling of facets in isotropic stones:

http://gemologyonline.com/Forum/phpBB2/ ... .php?t=802

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There appears to be an article written on internal strain in "The Rough Diamond Review" magazine. Perhaps you can get a copy.

http://www.roughdiamondreview.com/05June04.html

Good luck.

The article brings up an interesting topic. Internal strain can cause the effect of birefringence in diamond that can give the appearance of a false doubling of the back facets when the diamond is cut. Webster's Gems, 5th Edition pages 66.67.


The link provided is broken, and I don't have a copy of the book mentioned, so I was just wondering where I could read more about this "false doubling".

I appreciate the links/explanations on strain, I think I understand it now. I don't mind the geology-centric explanations either, roshanravan :)


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 Post subject: Re: Explanation of internal strain
PostPosted: Sat Jun 09, 2012 10:04 am 
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"The Rough Diamond Review" can be purchased here:
http://thediamondpages.com/


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