Firstly of the thousands of diamonds I have bought, sold and handled there have been very few perfect cut stones.
The different cuts.
W.R Eulitz. Published a work in 1972 "Determining mathematically the optimum proportions to achieve the optimum fire".
These calculations are very close to the "Practical fine cut". Set by Eppler in 1939 while examining exceptional fire and brilliance in diamonds he learned that cutters preferred quite definite size ratios and measurements. The cutter would look at the stone the size and weight then decide on the girdle diameter and cut the % size ratios whenever practicable.
This cut is the benchmark and preferred cut in Germany.
This cuts measurements only allows for light going into the stone perpendicularly. Light entering obliquely is not taken into account and a great amount of fire can not be expected from this stone. The crown is too high.
This is the standard cut in America and is the basis for cut grading in GIA Certifications. The fire meets the highest requirements.
Table size is very important, as this is where most of the light will travel into the diamond. The ideal table size to girdle diameter is between 55% to 60%.
There is a reasonable easy way of seeing if the table is around 60% of the diameter of the girdle. The path of the table edges form two squares. Look at the Picture to the right showing one of our stones in a mount and you can see 2 sides of the square. Which can be seen when looking at the face of the diamond. When the 4 lines in each square are straight the table is 60% of the girdle. When the lines are concave they are 65% and upwards as they become more concave. When the lines are convex the table is 56% and downwards as they become more convex.
Please note this is only used when the upper girdle facets and table facet sizes are proportionately the same size. Which is what they should be. Everything to do with cut grading diamonds are proportionate with sizes. The same as when you are dealing with a customer you have to weigh all the customers requirement up and pick the best stone for the customer. I want the customer to go home happily with their diamond and then wake up the next day after a good night's rest and be happier with the stone than they were the day before.
The girdle of the diamond is also important. This should be very thin to thin. It should be like a fine line to the naked eye. It should not be like a knife-edge as this may cause cleavages or nicks when the stone is set. If the girdle is too thick light comes out of the diamond through the side and is not reflected into the diamond to give it more fire.
You also have to check the conformity of the girdle so it is the same thickness at 12 points around the stone, as you don't want an irregular girdle. The girdle can be left natural, polished or faceted. There is not a size for an ideal girdle as thickness of the girdle is in proportion to the different size stones.
There are only 2 exceptions to this where you would want a slightly thicker girdle.This is on the ends of a marquise or pear cut at the points to give these areas more strength. This should not be thick, but slightly thicker than the rest of the very thin to thin girdle on the stone. The ideal thickness is twice as thick as the rest of a well cut girdle.
Estimate the pavilion depth with the table reflection.
This is a technique that is very well recognised and widely used. An experienced grader will be within 1% of the exact percentage. Basically you get different types of patterns that show on the face of the table with different proportions. These patterns change as the proportions change.
The art is knowing the exact pattern that tells you the stone has optimum fire because to have this optimum fire the internal characteristics are always the same (Relative to the size and make of the stone). The table is a viewing screen. It is the same principle as your own eyes where the image forms on the retina.
The pattern tells the experienced grader a lot more about the stone than you can ever imagine. I can look at a stone for 15 seconds and tell you a lot about the cut than than most people would ever be able to. You cannot tell what clarity grade a diamond is by looking at it with the naked eye unless it is a P or I grade.
I could carry on explaining about the cut but there are many different aspects not as yet explained, please see the list below for the main subjects. If you have any specific questions please e mail me and I will get back to you as soon as I am able.
The following are all flaws in the cut not in the stone when they appear on the surface of the diamond.