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 Post subject: Different hardness in one stone depending on cut?
PostPosted: Mon Feb 28, 2011 11:08 pm 
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Hi guys!

Just recalled something I noticed a few years back:
On a small cabochon ringstone I was scratch-testing its hardness, with steel, quartz, corundum and then diamond, trying to determine whether it's jade or not (I know scratch-testing is a rude way, especially on cut stones :oops: ). I noted that the stones scratched the underside easily, but the rounded side could barely be scratched even with the diamond :-s

That made me wonder, does the cut have influence on the resistance of the material / are rounded surfaces 'harder' than flat facets?


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 Post subject: Re: Different hardness in one stone depending on cut?
PostPosted: Mon Feb 28, 2011 11:54 pm 
Ratna wrote:
Hi guys!

Just recalled something I noticed a few years back:
On a small cabochon ringstone I was scratch-testing its hardness, with steel, quartz, corundum and then diamond, trying to determine whether it's jade or not (I know scratch-testing is a rude way, especially on cut stones :oops: ). I noted that the stones scratched the underside easily, but the rounded side could barely be scratched even with the diamond :-s

That made me wonder, does the cut have influence on the resistance of the material / are rounded surfaces 'harder' than flat facets?


Some crystals show differing hardness according to the direction of the scratch. Kyanite is the usual example given but it applies to other crystalline substances too. However, this crystal property is easily demonstrated with Kyanite and basic Mohs testing, as the variation of hardness is (from memory) from Mohs 6-7 on either the a or b axis to 4-5 if scratching parallel to the c axis.

That said I suspect that the main problem you contend with is the vectoring of the force you are applying to the stylus(and consequent skidding?) as a result of the curvature of the domed surface. Jade is not particularly hard, lying between 6 and 7.

There's more on hardness measurement in the archive if you want to chase it up.


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 Post subject: Re: Different hardness in one stone depending on cut?
PostPosted: Wed Mar 02, 2011 2:20 pm 
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Quote:
There's more on hardness measurement in the archive if you want to chase it up.


Spare us Owen,,,we're surely not going THERE again. :roll:


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 Post subject: Re: Different hardness in one stone depending on cut?
PostPosted: Wed Mar 02, 2011 3:22 pm 
Frank wrote:
Quote:
There's more on hardness measurement in the archive if you want to chase it up.


Spare us Owen,,,we're surely not going THERE again. :roll:


I'd gone off to sleep Frank, before you nudged to see if I was still awake.... :smt116 If you can't get off, yourself, should I get up and fix you a cup of cocoa?

Anyway, Ratna's curiosity will lead him 'THERE' - or not as the case may be. It was ten to five, at the end of a long night, but I hope that the answer given was not unduly repetitive and answering the question he asked in so far as it is answerable, if without elaboration.

Now, don't nod off before before I bring you up your cocoa or I shall be hurt... Two sugars as usual?


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 Post subject: Re: Different hardness in one stone depending on cut?
PostPosted: Wed Mar 02, 2011 7:10 pm 
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Kerensky wrote:
Frank wrote:
Quote:
There's more on hardness measurement in the archive if you want to chase it up.


Spare us Owen,,,we're surely not going THERE again. :roll:


I'd gone off to sleep Frank, before you nudged to see if I was still awake.... :smt116 If you can't get off, yourself, should I get up and fix you a cup of cocoa?

Anyway, Ratna's curiosity will lead him her 'THERE' - or not as the case may be. It was ten to five, at the end of a long night, but I hope that the answer given was not unduly repetitive and answering the question she asked in so far as it is answerable, if without elaboration.

Now, don't nod off before before I bring you up your cocoa or I shall be hurt... Two sugars as usual?

lols :smt083 :D

Thanks Kerensky for your info :wink:
I knew about kyanite, it's an interesting stone. Reminds me of the difference in resistance that wood exhibits depending on which direction you work it.

In my case I applied even more pressure on the domed surface after finding out it was so hard to scratch, and even then it didn't do much. It didn't really skid either, though maybe a bit, can't remember :-k It's surely a microcrystalline stone, so the kyanite-principle doesn't apply here......or does it?


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 Post subject: Re: Different hardness in one stone depending on cut?
PostPosted: Wed Mar 02, 2011 9:59 pm 
Ratna,

So sorry for the gender confusion. In cyberspace, all cats are grey :lol:

The appreciable directional hardness variation is present in varying degrees in many crystalline materials. In another thread, only today, PreciousGems was commenting on its noticable effect in Zircon cutting. I guess that gemmos make a fuss about Kyanite only because the effect is easily demonstrable using the crude Mohs methodology and scale.

In point of fact, when hardness is tested and recorded accurately using a rational scale of measurement (as is done by engineers and material scientists) the variation in force required to produce a scratch when testing Diamond in different planes can be more than 30 GigaPascals. To give you a perspective on that figure:
- The approximate directional hardness variation in Diamond from the most to the least is a step down of around 20%.
- "So little?", I seem to hear you ask.... Well, that 30 GPa variation is itself greater than the entire, maximum hardness of Corundum! I.e. 30GPa represents a greater change in hardness than would the substitution of Talc for Corundum.

And, yes, hardness can be measured in microcrystaline materials but, when more than one crystal is tested and the orientation the myriad microcrystals is haphazard, then there can be no directional variation in hardness observed.


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