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 Post subject: Re: gemstone hardness
PostPosted: Tue Sep 13, 2011 4:13 pm 
nirmala-raman wrote:
thanks for all the info ...can u help me with gems that have alexandrite effect.


Colour change stones? There's a *lot* of information here. Search the GO archive by enterring colour chanage into the on line search box in the left hand panel of the page you are looking at. Also, check out the Gemology Project (l/h panel again).

You can always come back for anwers to questions that you can't find in those places :)


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 Post subject: Re: gemstone hardness
PostPosted: Wed Sep 18, 2013 6:58 am 
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Hardness of gem is depends upon the substance you are mixing as impurities, it does not depends on the cutting style...


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 Post subject: Re: gemstone hardness
PostPosted: Fri Sep 27, 2013 2:56 am 
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Thanks for the information but, what should be the exact ratio of impurities for accurate hardness?


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 Post subject: Re: gemstone hardness
PostPosted: Sat Sep 28, 2013 10:41 am 
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By impurity, do you mean inclusions of other minerals, or the trace elements responsible for color.


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 Post subject: Re: gemstone hardness
PostPosted: Sat May 03, 2014 8:41 am 
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hardness is “the ability to resist scratching” and nothing more...


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 Post subject: Re: gemstone hardness
PostPosted: Sat May 03, 2014 1:03 pm 
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Exactly. Impurities as chromophores don't affect hardness, nor do inclusions really, as long as they're small enough. Now, a band of galena in your agate cab will affect hardness, because you have a big chunk of lead in your quartz that will undercut because it's so soft. But in gemstones, you're dealing with a single mineral, and any sizable impurities (like matrix) are cut out.

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 Post subject: Re: gemstone hardness
PostPosted: Fri Apr 24, 2015 1:20 pm 
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ttnt779 wrote:
i'm wondering if the hardness of gemstone is decided by the main element and impurity of the gemstone


It's more about the crystalline structure than the component elements. For example, garnets generally have the same formula (X3Y2(SiO4)3). Almandine (Fe2+3Al2(SiO4)3) has about the same hardness as Pyrope (Mg3Al2(SiO4)3).

Even if you consider silicon (Si), or even Si0x, to be the main element, quartzes (SiO2) are a bit softer than most garnets.

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 Post subject: Re: gemstone hardness
PostPosted: Sun Apr 26, 2015 6:48 pm 
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It's more about the crystalline structure than the component elements.

It's both. You can't have one without the other. When you name it, you define both.


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 Post subject: Re: gemstone hardness
PostPosted: Sun Apr 26, 2015 7:23 pm 
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It's probably fair to say that structure is more important in a general sense, though. The most extreme change from a structural difference I can think of is diamond vs graphite, while the biggest difference within a structural group by composition is andradite (6.5ish) vs. YAG (8.5ish). That's still orders of magnitude on most other scales but still. Can anyone else think of a more extreme example? There's certainly a nice bit of variation in the clinopyroxenes (~5.5-7), and no doubt the amphiboles do weird things as per usual.

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 Post subject: Re: gemstone hardness
PostPosted: Sun Apr 26, 2015 11:14 pm 
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sapphires, from my area, can have dramatically different hardness within one piece of rough.
No visible reason, but it can take minutes to cut a facet, using 1500 lap.
The next one will overcut in a split second.
Yellows are often vexatious like this!
Barry.


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 Post subject: Re: gemstone hardness
PostPosted: Sun Apr 26, 2015 11:38 pm 
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Please elaborate, Barry.


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 Post subject: Re: gemstone hardness
PostPosted: Mon Apr 27, 2015 4:54 am 
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Barbra, it is well known amongst gemcutters on the Qld fields, that our sapphires have dramatic hardness changes, within some stones.
There is absolutely no way to tell where they are, until you start faceting.
I have tried to find a reason, by carefully loupeing the gem, when I hit one of these areas.
The only thing I've found, so far, is there is often a colour zoning associated with the extra hardness. This can be very, very subtle.
Another time I find this, is when cutting synthetic material, and using the "rind" of the boule, in the design.
This afternoon, cutting some vanadium dopped, colour change corundum, I placed the "rind",(deep purple), under the pavillion end facets, on an oval. The facets took about 20 times longer to set, than any of the others, using the same 1500 lap.
Earthling will have more to contribute on this subject, I'm sure!
Barry.


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 Post subject: Re: gemstone hardness
PostPosted: Mon Apr 27, 2015 8:48 am 
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due to the start of micro-cracks in the basal twinning and pyramidal slip´s ? http://www.msel.nist.gov/lawn/Publications/Papers.BRL/PapersBRL.1988/JACS%2071%2029%201988.pdf

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 Post subject: Re: gemstone hardness
PostPosted: Mon Apr 27, 2015 11:12 am 
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I've had what seems to be directional hardness issues with sapphire, but they were more noticeable on the synthetics.

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