CIBJO releases Gemmological Special Report: considers process of separating measurable facts from opinion; See Gemological Articles below.
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 Post subject: Diamond proportions: Discussion after Quiz
PostPosted: Thu Dec 15, 2011 6:20 pm 
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Can you recognize the quality of diamonds proportions using only a loupe, in face-up view?

Here is my quiz. Seven diamonds, you have to say what cutting errors they have, and give reasons for your answers.

Please don't post your replies here, send me PM or e-mail!

Deadline: Sunday, 25, 2011 (hope somebody will have one more present for Christmas! :D )

Prize for the best answer, with correct identification in all stones and best explanation - this stone.
Not diamond, but similar, and brilliant cut too! :D

(images made with DiamCalc software)

So here we go:

1.
Image


2.
Image


3.
Image


4.
Image


5.
Image


6.
Image


7.
Image

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 Post subject: Re: Diamond proportions quiz with prize!
PostPosted: Thu Dec 15, 2011 6:24 pm 
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Well someone will get a FANTASTIC gift this Christmas! But it won't be me! LOL

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 Post subject: Re: Diamond proportions quiz with prize!
PostPosted: Thu Dec 15, 2011 6:28 pm 
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Oh, man! That is a great prize! Just wish I had time to participate :( .

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 Post subject: Re: Diamond proportions quiz with prize!
PostPosted: Thu Dec 15, 2011 9:07 pm 
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Can someone pop around to check on Egor's well being, he just received my attempt. :smt085

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 Post subject: Re: Diamond proportions quiz with prize!
PostPosted: Fri Dec 16, 2011 12:38 pm 
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Don't panic Mehoose!
You are a temporary winner so far I only have your answer! :wink:

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 Post subject: Re: Diamond proportions quiz with prize!
PostPosted: Thu Dec 22, 2011 4:38 pm 
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Very few answers up to the date!

Hey, it's not so difficult!

Three days left... [-(

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 Post subject: Re: Diamond proportions quiz with prize!
PostPosted: Sat Dec 24, 2011 5:59 am 
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Hi guys...!!

i just did my part!

and i'm nervous as we used to be after appearing for our school exams! [-o<
:lol:

-Akshat!


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 Post subject: Re: Diamond proportions quiz with prize!
PostPosted: Sat Dec 24, 2011 12:01 pm 
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I don't get it. :(

#-o Maybe that's because I look at old mine cuts all day long.


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 Post subject: Re: Diamond proportions quiz with prize!
PostPosted: Sat Dec 24, 2011 3:45 pm 
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Hehe, and I feel I do not qualify as competitor as I'm a moderator...
Now I remember why I like OMCs, OECs and ancient cuts in general more than the boring round brilliant cut, ideal cut or whatever they are called :)

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 Post subject: Re: Diamond proportions quiz with prize!
PostPosted: Sat Dec 24, 2011 8:50 pm 
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Barbra your answers couldn't be worse than mine. :oops: :lol:

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 Post subject: Re: Diamond proportions quiz with prize!
PostPosted: Mon Dec 26, 2011 10:39 am 
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TIME OVER!

And the prize goes to …. Well, actually the prize stay at home, nobody sent all correct answers to this quiz! :lol:

Only four answers received and the winner is …. - Paco Corodia (Barcelona, Spain), with 5.25/7 score. Four totally correct answers (1 poin for each), one nearly correct (0.75 points), one partly correct (0.5 points) and one incorrect (0 points). =D>

Very close to him - Mikko Åström (Finland) – 5/7, four totally correct answers, two partly correct and one incorrect, and also good explanation of all cases. =D>

akki4i (India) – good attempt, but only 3/7 score. =D>

Mehoose (Australia) – I think you need some more practice with diamonds! :smt065 But thanks anyway for your attempt! =D>

Now, here are the correct answers and explanation.
(The pictures below are made by me for IGE diamond grading course using DiamCalc software).

This quiz was to check your ability to grade basic brilliant cut proportions in face-up view, these are:

- Table diameter
- Crown height (or crown angle)
- Pavilion depth (or pavilion angle)


To grade the quality of diamond cut you should also check girdle thickness, culet size and finish (symmetry and polish), but these characteristics were out of this quiz.

To see ideal brilliant just have a look at the picture 5. The proportions of this stone are:
-Table diameter – 57.5%
- Crown height (crown angle) – 14.6% (34.5º)
- Pavilion depth (pavilion angle) – 43.1% (40.75º)

In all other stones only one of these parameters was changed to very incorrect values, corresponding to low proportions grade.

1. The most difficult one, both Paco and Mikko have bad answers in this stone, thinking it’s totally correct. The problem of this stone is low crown height and crown angle (11.8%, 29º). Very common cut defect.

To check it you have to look at the “arrows” formed by pavilion mains, if the crown angle is correct, the “heads of arrows” seen through the crown mains should be twice bigger than the same facet seen through the table (see picture).

Image


2. Everything Ok, but star facets on the crown are too large (62%), they should be around 50% of the distance between table border and girdle (see picture). This deviation can cause you visual impression that the table is too small (see point 3), but in fact the table here is totally correct (57.5%, same value as in the picture 5 and other pictures with correct table diameter – all but 3).

Image


Remember: if you use the visual trick of “two squares” for table diameter estimation, as explained in point 3, don’t forget to check that the stars size is correct. If not, this estimation won’t work, with the same diameter of the table you’ll have “rounded squares”, convex or concave.

Image


3. Very common cut defect - table too large (67%). You can see it looking at “two squares” formed by the table and eight star facets (see picture below), totally rounded in this case. (Don’t forget to check the stars size if you use this method, as explained in point 2!)

Image


4. Very common cut defect – pavilion too deep (46%, angle 42.61º).
To check it, look for the position of stars reflections on pavilion mains (see picture).

Image


5. Ideal proportions diamond picture.

6. Typical “fish-eye” effect caused buy too shallow pavilion (37%, 36.5º in this case). You can see a circle formed by girdle reflections on pavilion facets. Girdle is faceted in this rendering, the effect is more pronounced in real diamonds if the girdle is not faceted or polished, that causes white circle on dark pavilion lacking of brilliance, similar to “fish-eye”.

7. Everything is Ok with basic proportions, but lower half facets are too short (65%), while optimum value is between 75-85%. Very common characteristic of old cut diamonds and also some early cut brilliants with correct proportions but too short lower halves.


Well, we are in Christmas time when miracles are possible!
Although not all the answers were totally correct, the prize will be sent to the best score participant – Paco Corodia from Barcelona! =D> =D> =D>

Image

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 Post subject: Re: Diamond proportions quiz with prize!
PostPosted: Mon Dec 26, 2011 1:45 pm 
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Wow.....looks like Mehoose and I will be spending a little extra time freshening up.
:?


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 Post subject: Re: Diamond proportions quiz with prize!
PostPosted: Mon Dec 26, 2011 1:51 pm 
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Congrats Paco =D> =D> =D>
Happy Xmas to Barcelona. I visited your beautiful city couple of months ago (my first live Barça match).

Stone number 1: :oops: :oops: :oops:

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 Post subject: Re: Diamond proportions quiz with prize!
PostPosted: Mon Dec 26, 2011 2:22 pm 
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:) this was fun!!

and yes, i've already started looking for books! lol
:idea:


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 Post subject: Re: Diamond proportions quiz with prize!
PostPosted: Mon Dec 26, 2011 7:55 pm 
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YAY, I won the wooden spoon! :lol:

Thanks for the explanations Egor.

You've got that right Barbra, I've never worked with diamonds so it's a good starting point. I've always been a visual first before I could even consider getting the technical...if at all :wink: If it looks good it is!

Thanks for the fun Egor and Congrats Paco. :P

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