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PostPosted: Wed Jul 30, 2008 7:09 pm 
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Doos wrote:
Hi,

Reference was given on the GP, please read the article.


I did. The article giving the ratio of 40 has no references. The other article
giving a ratio of a ratio, references Read, which I don't have. If you know the answer, why not tell?


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PostPosted: Wed Jul 30, 2008 8:08 pm 
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Hi,

I always share what I know the best I can, as do most people here. Usually you can accept what Read writes. If you like to know his sources then I suggest you ask him. It is not a subject that keeps me occupied at night.

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PostPosted: Thu Jul 31, 2008 10:22 am 
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I'll add this for everyone's consideration.

The following is a quote from the GIA's coursework.

"In fact, destructive scratch tests done with a diamond-tipped, weighted lever show that diamond is 140 times harder than corundum and 124,000 times harder than talc. These tests are done only rarely in gem labs, and should never be done on transparent, finished stones."

We should consider what hardness means only in terms of Gemology and how it is Gemologically tested. Resistance to scratching on a non-linear scale.


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PostPosted: Sun Aug 22, 2010 10:09 pm 
JB wrote:
I'll add this for everyone's consideration.

The following is a quote from the GIA's coursework.

"In fact, destructive scratch tests done with a diamond-tipped, weighted lever show that diamond is 140 times harder than corundum and 124,000 times harder than talc. These tests are done only rarely in gem labs, and should never be done on transparent, finished stones."

We should consider what hardness means only in terms of Gemology and how it is Gemologically tested. Resistance to scratching on a non-linear scale.


With all due respect, hardness is hardness and Mohs scale is a poor way to measure it. For the hardness of diamond, Mohs scale has actually become meaningless since that scale relies implicitly on the fact that diamond is the hardest material known. This has actually been untrue since the late '70s - that's now 30+ years ago - when it was discovered how to create a form of Fullerite that is harder than diamond. Since then nanorod diamond (another fullerite derivative has been developed and ultrahard fullerite has been (for near 30 years!) the preferred material for testing the hardness of the natural diamond allotrope of carbon. Presently, the scientific community expects that, in the next few years, at least three other materials are likely to prove harder than natural diamond. These are:
- A form of Lonsdaleite (another allotrope of Carbon).
- W-Boron Nitride
- Rhenium Diboride

To measure properly the hardness of a material requires carefully controlled test conditions. In general, these are:
- Testing is performed at NTP.
- The surface of all substances tested has the same form factor (normally planar)
- The material of which the testing stylus (or other form of test head) is fully described, both in its composition and in its shape.
- The test head shall always be brought to bear on the test surface in a manner where:
(a) There is very little relative velocity between the test head and the surface under test at the moment of contact between them.
(b) The hardness of the test material is determined at the point where a known amount of force applied to the test head shall cause a scratch (form of cracking) in the test surface or where the surface of the material under test shall be permanently distorted by the force applied (i.e. the force applied is just sufficient to distort the shape of the test material past the point up to which the inherent prorperty of elasticity in the test material would return the test surface to its original shape when the testing head was removed.
(c) The test head shall be bear against the test surface so that force is applied to the test surface normal to the plane of the test surface.
(d) Hardness is measured as the requisite force applied to a test head and brought to bear on a known area of the material under test.

There's one other important condition for the measurement of hardness. This is, the material of which the test head is made, *must* itself be measurably harder than the hardness of the material under test. Where this condition is not observed (i.e. the testing and tested materials are equally hard), very anomalous (read worthless) measurements of hardness are obtained. Of course in those dear dead days when no one knew of any material harder than diamond, this meant that testing - by whatever method - of the hardness of natural diamond produced anomaloius results. This problem is still with us but the point of concern has shifted. The hardness(es) of natural diamond have long since been properly determined but there is now real difficulty in showing empirically (by practical testing) the true hardness of the new super-hard materials as listed above.

Mohs scale has exactly ten *non-rational* but sequential numeric values in it. No more and no less. As was illustrated in this forum a while back, decimal points can only can only be interpolated into the Mohs scale by measuring the hardness of a substance on a rational scale, such as Vickers or Knoop, and then, by X-Y graphing ar some other method, to transpose this value onto the Mohs scale.

Do or should Gemmos care? I think the answer is both No and Yes :-)

- No because, thus far, Gemmos have little use for knowledge of hardness as a scientific and rational measurement of a property of materials, concerning themselves only with knowing for a quite small number of materials that, on an arbitrary and non-linear scale numbered 1 - 10, which of these materials is harder, softer or about the same as the hardness of some other.

- YES. Because gemmos are people with curious and logical minds and like to pose questions, such as 'Well....by how much is diamond *actually* harder than corundum?'. To answer such a question requires understanding something about several scientific testing methods for hardness (none of them Mohs) and their relative advantages and limitations. If this was always done, (by including it in gemmo training?) the more 'anomalous' statements found in some books of the Gemmo Bible would be quietly weeded out and decently buried. Scientific knowledge constantly moves on - and gemmo education should in every respect and not just in most respects - move on in step with the advances in materials scientific knowledge.

So.... how to wind this rant up with a proper gemmo bang?....

Rationally tested by modern scientific method, natural diamond (and many other crystalline substances) are shown to be possessed of no single measure of hardness. In the case of natural diamond there are two equally valid values. The value for diamond 111 is sharply different from that obtained in testing diamond 110 (from memory, the difference in the two values is 30+ GPa - which is quite a lot. It follows that a true statement for the hardness of natural diamond would be something like 131-167 GPa, depending of the direction of the test.

In the simplest terms, natural diamond is known to be about 4-5 times as hard as corundum.

Phew! :twisted:


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 Post subject: Re: Re:
PostPosted: Mon Aug 23, 2010 3:25 am 
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Kerensky wrote:
Phew! :twisted:


now mate, take a deep breath and have a glass of wine, you must have dry throat... :wink: :lol:
ciao
alberto

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 Post subject: Re: Hardness, relationship corundum-diamond
PostPosted: Mon Aug 23, 2010 4:05 am 
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:lol:

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