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 Post subject: gemstone hardness
PostPosted: Sun Sep 11, 2011 1:11 pm 
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=D> hi,i was wondering if presence of impurities lowers the hardness of gemstones.like is a carbochon of the same family of lower hardness than the facetted gemstone.


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 Post subject: Re: gemstone hardness
PostPosted: Sun Sep 11, 2011 3:08 pm 
[-X [-X Nopes, the difference betweend hardness of a mineral doesn't occur if the stone is cut into a faceted or a cabouchon.
The inclusion doesn't affect the hardness of the stone either(correct me if I am wrong), unless the inclusion is at the exterior part of the stone. Like if graphite(Moh's Hardness : 1.5) inclusion is tested, which is at the exterior of a diamond(Carbon hardness on Moh's Scale : 10) then the results may vary, otherwise not ! :wink:


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 Post subject: Re: gemstone hardness
PostPosted: Sun Sep 11, 2011 4:06 pm 
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an inclusion isn't necessarily the same substance as the one in which it is trapped, that should make it obvious that the hardnesses of the two substances would be different. Inclusions that are not mineral inclusions (feathers, negative crystals, etc.) might affect the durability of a stone, but not the hardness. Durability is a substance's ability to stand up to chipping, breaking and heat/chemical attack. Having many inclusions inside of a stone might weaken the bond of the structure making it more prone to breakage, like emerald. Hardness is a substance's ability to be scratched. The type of cut would have extremely little, if no effect on the hardness of a stone.

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 Post subject: Re: gemstone hardness
PostPosted: Sun Sep 11, 2011 4:44 pm 
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thanx and how accurate is a presidium gem tester..i have recently purchased one...can i depend on its results...i am a collector...is there any other instrument which will help me recognise gemstone rough.... :lol:


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 Post subject: Re: gemstone hardness
PostPosted: Sun Sep 11, 2011 5:45 pm 
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A Presidium tester is a useful tool in the right hands to narrow down potential identification suspects. Emphasis on a useful tool in the right hands. It is not the do-all end-all for people wanting to to know what they have. I suggest you educate yourself in gem identification and purchase the proper tools (and education) if you want to begin identifying your own rough. This forum is a great resource for education- check out the links down the left-hand side of the page. Members here are always willing to help as you learn . Otherwise, it might be wise to invest in a professional Gemologist for your identification purposes.

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 Post subject: Re: gemstone hardness
PostPosted: Sun Sep 11, 2011 5:55 pm 
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A few notes on hardness testing:

Of course, in a scratch test, it is much more practical to scratch the glass using an angular part of the stone (this is common sense).
But the type of cut certainly doesn't affect the hardness of the mineral.

However there are some minerals with strongly anisotropic hardness so that they will yield different hardness results depending on the direction of the scratching. The best exemple is kyanite: hardness is 5.5 parallel to [001] and 7 parallel to [100]

Also note that rocks are mixes of different minerals, so a rock might yield different results in a scratch test depending on which constitutive mineral actually scratched the reference material. For instance, granite associates quartz (H=7) and feldspar (H=6)

Same thing with minerals associations, for instance calcite (softer than glass) with some pyrite dust (harder than glass) over it.

It is also possible that a mineral aggregate that lacks cohesion might test low hardness, for instance pyrolusite hardness is 6 in individual crystals, but aggregates can be as soft as 4 or 5 and massive or earthy forms will mark paper and leave powder on fingers (a hardness under 2).

At last, keep in mind that a single mineral specy can show different hardness depending on the tested sample: for instance hematite hardness is 5 to 6 depending on the sample (I guess this is because defects in the crystal structure may occur, thus the hardness may vary a little bit from a sample to another)


Last edited by cascaillou on Sun Sep 11, 2011 6:49 pm, edited 1 time in total.

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 Post subject: Re: gemstone hardness
PostPosted: Sun Sep 11, 2011 6:38 pm 
nirmala-raman wrote:
=D> hi,i was wondering if presence of impurities lowers the hardness of gemstones.like is a carbochon of the same family of lower hardness than the facetted gemstone.


Hi and welcome :D

Your seem to be mixing two separate things (if I understand correctly):
- Whether a hardness measurement is affected by the cut of a gemstone. It is not.
- Whether impurities in the crystal affect the hardness of a gemstone. Yes, they may. Not if present as observable inclusions but yes, if incorporated into the crystal lattice itself. Since gemmos only use the very crude Mohs's scale of hardness this probably is only detectable by gemmos in the 1-2-3 Mohs hardness range.

Another comment here has been on testing hardness on a facet junction as opposed to on a plane surface. In short, for any rational scale of hardness measurement (e.g. force applied to a specified surface area by a stylus of specified dimensions). The testing tool and methodology must be absolutely uniform between all tests - or else the results are - at the least - suspect. If you are interested, you can firstly read just a little more here http://gemologyonline.com/Forum/phpBB2/ ... 5&start=15 . You could follow this up by researching other methods of hardness testing, other than the scratch testing against a Mohs's scale of hardness that *only* rates 10 minerals in a rough order of hardness and with all others falling somewhere in between.

Most importantly, never forget that the difference in hardness between Mohs 1 and 2 is simply nowhere approaching the difference in measured hardness there is between Mohs 9 and 10. Even the difference in harness between Mohs 8 and 9 is nowhere near the same as the difference between Mohs 9-10. If you wish, you can google up a lot of academically sound information of the hardness testing of materials and the various testing methods in use by those who need better than an 'order of mineral' quality of result.


Last edited by Kerensky on Mon Sep 12, 2011 1:04 pm, edited 1 time in total.

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 Post subject: Re: gemstone hardness
PostPosted: Sun Sep 11, 2011 11:53 pm 
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thank you very much ......actually i have recently started collecting minerals and read all i can but still when i get to actually look at a rough other than the common gemstones and well formed crystals i am lost....i had come across some pieces of alexandrite,they seemed to have good color change from green to pink red but when trying to cut them they seemed to just chip off..it was included ....what are the other minerals that change color like alexandrite.....i am posting pictures of the rough...it seems to be that my pic file is too big....i post the pics once i correct this..i am from india and these alexans are from orissa.


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 Post subject: Re: gemstone hardness
PostPosted: Sun Sep 11, 2011 11:58 pm 
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See this thread on posting pictures:
viewtopic.php?f=17&t=3679


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 Post subject: Re: gemstone hardness
PostPosted: Mon Sep 12, 2011 1:27 am 
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hi,i am full of questions,i know that large sized aquamarines are quite common but do they occur inclusion free.what is etching on the surface of an aquamarine...is it common to find large aquas of say 1/2 kg and more inclusion free... what is the difference between a citrine and a topaz....other than refractive index and specific gravity are there more simple ways of differentiating them in the rough form..thanx


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 Post subject: Re: gemstone hardness
PostPosted: Mon Sep 12, 2011 11:25 am 
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as it comes to minerals, just see as many as possible through reading illustrated books, browsing mindat photo galleries and especially by visiting mineral shows. At some point your eye will be used to the many possible morphologies for each of the most encountered minerals. Note that besides careful visual observation of the mineral itself, mineral associations are also a good key to identification, and knowing details about the location of origin can significantly facilitate researches.
Past that point, you will still need some mineralogical/gemmological testing (i.e. physical and optical tests) to elmininate possibilities when there's a doubt between several possible candidates for a specimen. Of course, the more you read and learn, the more competent you get.


Last edited by cascaillou on Mon Sep 12, 2011 8:09 pm, edited 5 times in total.

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 Post subject: Re: gemstone hardness
PostPosted: Mon Sep 12, 2011 1:32 pm 
Cascaillou is quite right. the more you read and the more you look, the more you will come to know.

In this forum we mainly consider gems only and those parts of mineralogy that that are necessary to properly understand gems and be able to ID them. There are only about 100 gem crystals. But, since in gemmology, the same stone can be considered to be several different gem varieties, sometimes according to colour difference only, this and adding in some rarely-cut curiosities, means there are about 550 gem varieties discussed, collected etc by gemmologists. This is but a small fraction of the number of known minerals.

Do you want you learn just about gems or about all minerals? For the study of gems, you can do no better to start than to follow and participate in forum discussions here. But if your interest is in mineralogy, you would probably be better off following the MINDAT forum. Of course, there's no reason you should not do both :D


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 Post subject: Re: gemstone hardness
PostPosted: Mon Sep 12, 2011 5:08 pm 
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Quote:
Of course, there's no reason you should not do both

Kerensky is right, I'm into both gems and minerals, and I feel ok

luckily, gems are actually minerals, otherwise that couldn't be without some kind of schizophrenic break in my personality :lol:


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 Post subject: Re: gemstone hardness
PostPosted: Mon Sep 12, 2011 7:39 pm 
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here's one for you nirmala if you wanna train (but, at least at first, don't read the stickers at the beginning of the post, and don't look at other people guess!):
http://www.geoforum.fr/topic/21575-a-ch ... algache-1/
These 16 specimens are from madagascar. You guess what each of these is.


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 Post subject: Re: gemstone hardness
PostPosted: Tue Sep 13, 2011 11:46 am 
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thanks for all the info ...can u help me with gems that have alexandrite effect.


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