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Arithmetic Off On Reported Measurements. Does This Change Anything?


Mainer
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Since I don't have real diamonds to look at I tend to obsess a bit on the reported numbers and I have found that the measurements of the diamond diameter and depth don't always add up.  

 

So, if a diamond's measurements are 7.63-7.66 x 4.67   the depth%  should be 61.1%,  however,  it is reported as 61.2%.   If  61.2% is used in calculations wouldn't it throw off the calculations for the crown and pavilion angles  making one or both angles slightly more acute.  How much does this change what is reported about the cut and perhaps the reported quality of the cut?

 

On the other hand the angles may be measured separately and have nothing to do with a slight miscalculation of the depth of the diamond.  (I have got to get to a diamond show and learn more!)

Edited by Mainer
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Depth percentage is calculated based on the shortest width, not the average width.

This does bring up a point I try to explain to those who really get caught up in the numbers though.  The numbers on a report of any kind are only as good as the latest calibration.  I have seen stones come back from both the GIA and AGSL with just slightly different numbers and yet it is the same stone.  So this begs the question: which ones are correct?

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Crown and pavilion angles can't be reliably calculated from depth percentage. 

In the case of rounds, GIA uses average diameter to calculate depth percentage.  With fancies they use the minimum width.  In each case there's equipment calibration issues as well as rounding issues. 

 

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Interesting, where did you learn that?   We have seen inclusions that appear to be feathers but the reports only mention crystals.  I wonder what logic is being used since the GIA defines a crystal as a mineral contained within a diamond (which a feather is not) whereas a feather is a break in the structure. 

http://4csblog.gia.edu/2013/diamond-inclusions-defined

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Interesting, where did you learn that?   We have seen inclusions that appear to be feathers but the reports only mention crystals.  I wonder what logic is being used since the GIA defines a crystal as a mineral contained within a diamond (which a feather is not) whereas a feather is a break in the structure. 

http://4csblog.gia.edu/2013/diamond-inclusions-defined

George,

 

The logic is that any break that's entirely internal was caused by something internal.  The assumption is that it's a microscopic inclusion of some other material and that the 'feather' is the result of growth changes or thermal activity during the growth process.  That is to say, a crystal.  Weird, I know, but they get to write their own rules.  By definition, to be called a feather it must reach the surface.

 

I got it from the advanced diamond grading lab class I took from them so this came straight from the mouths of the folks training the lab graders.  I've gone so far as to ask the senior management at the lab about it because it's so counter-intuitive.  

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"And the numbers, at least the dimensions, don't tell you nearly as much as you think they do. "

 

That's hard news for someone that is looking for the magic number. :-)

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