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 Post subject: Hardness, relationship corundum-diamond
PostPosted: Sat Sep 08, 2007 12:09 pm 
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I have looked at the following links:
http://gemologyonline.com/mohs.html
http://www.gemologyproject.com/wiki/ind ... ness_scale

On both pages it says that Diamond is 10 times harder than corundum, and what is the reference to that statement? We know that using Rosiwal's scheme diamond is 140 times harder than corundum (diamond =140k, corundum=1k)... just curious, as it seems I must have forgotten other types of hardness schemes.


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PostPosted: Sat Sep 08, 2007 12:16 pm 
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Quote:
On both pages it says that Diamond is 10 times harder than corundum, and what is the reference to that statement?

Hi,

Could you qoute, as I can't see that statement on either page.


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PostPosted: Sat Sep 08, 2007 12:26 pm 
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Sorry typo from my side (not 10 it should be 40), it says 40 times, but that is also interesting since according to rosiwal it is 140...


http://gemologyonline.com/mohs.html wrote:
Mohs Scale of hardness is a RELATIVE scale, not proportional. I mean by this that a mineral with the hardness of 8 will NOT be twice as hard a a 4. (For example, diamond is 40X harder than sapphire!).It is really a scale of relative "scratchability".


and the quote from gemology project
Quote:
The difference in actual hardness between diamond and corundum is 10 times higher than the difference between talc and corundum.


According to Rosiwal; talc=0,03 and corundum=1000, diamond=140000)...

I am just curious what the referense data is used.


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PostPosted: Sat Sep 08, 2007 12:33 pm 
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Hi,

There are many hardness scales, Mohs, Vickers, Rosiwal, Knoop.
All of them use different parameters.
In gemmology the resistance to withstand abrasion is, mosty, used.

It is not very useful to measure the resistance on a gemstone by a steel ball in our trade.
The different data are merely used to put things in perspective.

Hope that helped.


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PostPosted: Sat Sep 08, 2007 12:50 pm 
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How did Mr Rosiwal come to the different hardness (ability to resist abrasion)? The ones you mentioned seem to be much about plastic deformation (metalls for example) and perhaps of no practical use when applied to gems.

I was just curious what the reference data came from. It cant be Rosiwal so it must be something else that I have missed. But I definitely agree that the reader gets the point that Mohs scale of hardness is realtive when giving descriptions like 10 times or 40 times harder, but perhaps other than me are wondering where those figures emanate from.

I sometimes exhibit the need to know where the data is coming from, not just in gemology.

Have a nice weekend.


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PostPosted: Sat Sep 08, 2007 12:59 pm 
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I sometimes exhibit the need to know where the data is coming from, not just in gemology.

Hi,

Consider that your homework for the weekend. We expect a full report by Monday.


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PostPosted: Sat Sep 08, 2007 2:11 pm 
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Well, I am also not that very interesting in reinventing the wheel so to speak, so if it is alright, wouldn't it be pretty nice presenting me with the reference when stating that diamond is x10 harder on one page, and x40 harder on the other.

Yes, there are a couple of different methods trying to establish a minerals (materials) hardness, like you said - knoop amongst others.

However, looking at knoop scale:
Corundum (Al203) with 9 on Mohs has 2100 knoop
Diamond (C, Type IIa) with 10 on Mohs has 9000 knoop
I dont see any ratio here that can be applied to the quotes previously mentioned.

Ok, lets look at Vickers (using the sclerometer using standard load);
Corundum 400 vickers
Diamond 1500 vickers
--> Cant seem to find the ratio here aswell...

Well diffent kind of loads could be used, but do the ratio change that much to get the numbers quoted above?

Whats left.... well I dont know, perhaps you do have to give me an F on this assignment, I cannot see any other option than leaving this topic (although it is sad not knowing where that data is coming from - but I also tend to choose fishing infront of getting answers sometimes).

I would indeed be very pleased if someone could enlighten me in this case. The world will not fall apart if no answer will be presented, but when saying something like A is X times harder than B and not telling C (under what circumstances hardness being tested) perhaps doesnt tell the whole story... well what do I know - I am just an old golddigging fool to begin with....


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PostPosted: Sat Sep 08, 2007 2:39 pm 
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Hi,

I'm not sure what you are driving at, but maybe you missed the "not a linear scale" part of it all.
Thanks for bringing it to our attention and in time we will rewrite it to make it less confusing.


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PostPosted: Sat Sep 08, 2007 5:35 pm 
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Quote:
It should be noted that this is not a linear scale. The difference in actual hardness between diamond and corundum is 10 times higher than the difference between talc and corundum.


No, I don't think I missed that part, as the text states Mohs is about relative hardness and not a linear scale, that "actual" hardness (whatever hardness method chosen) between the minerals differ.

What I am interested in is how the conclusion is made that the difference in actual hardness between diamond and corundum is 10 times higher than the difference between talc and corundum. Looking at Rosiwal or Knoop doesnt make sense.

Abrasion yes, but if not Rosiwal what hardness method being used to get the actual hardness comparison here?


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PostPosted: Sun Sep 09, 2007 8:07 am 
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Hi,

As I said earlier, those statements are just to put things in perspective. I have little interest in persuing the subject as I just don't really care much about it.
You are more than welcome to investigate and report of course.


Last edited by Doos on Sun Sep 09, 2007 10:28 am, edited 1 time in total.

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PostPosted: Sun Sep 09, 2007 12:33 pm 
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Then please delete this thread, it seems there is no verifyable answer to my question regarding the relationship in hardness between corundum and diamond being stated in the written texts on the urls I quoted..

Bright light what a sight and delight!


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PostPosted: Sun Sep 09, 2007 12:35 pm 
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Of course I'm not going to delete the thread.
You often bring up points of contention and then drop the ball like a hot potato.
If you would like to prepare a definitive explanation of relative hardness, Demantoid, with quantitative and qualitative examples of various schemes of actual or theoretical values, I would be happy to add it to our explanation on Mohs Scale of Hardness, with appropriate credits, of course.


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PostPosted: Wed Jul 30, 2008 3:16 pm 
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demantoid wrote:
Then please delete this thread, it seems there is no verifyable answer to my question regarding the relationship in hardness between corundum and diamond being stated in the written texts on the urls I quoted..

Bright light what a sight and delight!


Demantoid,

The Mohs scale is not a quantitative scale. The absolute hardness measured of Diamond via different tests is close 3.7 times (approx average) more than the absolute hardness of a Corundum. I agree with the same. I do not think Diamond is 10 times or 40 times harder than corundum.

It is best to define hardness here - it is the property of a given material to resist any permanent deformation. Mohs scale is a relative scale based on 10 different substances stating one is harder than the other. All the rest fall in between in decimal places. It does not specify how much pressure can a diamond or a corundum take without being punctured, abraded, split, scratched, etc. depending upon its structure.

An absolute test puts a value in Pressure applied and calculated with some formulas. This is dependent upon the substance used for abrasion or indentation in the testing equipment. For example, Knoop's apparatus uses a diamond of rhombic shape and a pyramdical head.

Hope this helps in this old thread.


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PostPosted: Wed Jul 30, 2008 4:41 pm 
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Barbra Voltaire wrote:
Of course I'm not going to delete the thread.
You often bring up points of contention and then drop the ball like a hot potato.


I don't blame Demantoid for getting frustrated. If I can summarize,

1. The reference given says that diamond is 40 times the hardness of corundum.

2. The question is simple: where did this number come from?

He's asked that question several ways, but failed to get an answer. Who wrote the article? They should know how the number was derived. If it was just off the cuff, say so.


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PostPosted: Wed Jul 30, 2008 4:50 pm 
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Hi,

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

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