Post subject: Diamond proportions: Discussion after Quiz
Posted: Thu Dec 15, 2011 6:20 pm
Gemology Online Veteran
Joined: Sun Oct 04, 2009 11:18 am Posts: 595 Location: Madrid, Spain
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! )
Prize for the best answer, with correct identification in all stones and best explanation - this stone. Not diamond, but similar, and brilliant cut too!
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
Post subject: Re: Diamond proportions quiz with prize!
Posted: Mon Dec 26, 2011 10:39 am
Gemology Online Veteran
Joined: Sun Oct 04, 2009 11:18 am Posts: 595 Location: Madrid, Spain
TIME OVER!
And the prize goes to …. Well, actually the prize stay at home, nobody sent all correct answers to this quiz!
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).
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.
akki4i (India) – good attempt, but only 3/7 score.
Mehoose (Australia) – I think you need some more practice with diamonds! But thanks anyway for your attempt!
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:
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).
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).
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.
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!)
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).
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!
Post subject: Re: Diamond proportions quiz with prize!
Posted: Mon Dec 26, 2011 7:55 pm
Gemology Online Übergoddess
Joined: Mon Oct 06, 2008 6:13 pm Posts: 5077 Location: Australia
YAY, I won the wooden spoon!
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 If it looks good it is!
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