61%

(In honor of Mark Osborne)

Question: Can a diamond whose crown angle is within tolerance of being a Class I or II, whose girdle thickness is neither very thin nor very thick and whose pavilion angle is also within tolerance of being a Class II actually be a Class III?

Answer: Yes, it is possible for the parts of a diamond to meet Class II tolerances but whose total exceeds the “61% Factor”. The 61% factor is when the crown height, girdle thickness and pavilion depth exceed 61%. Mathematically, 61% is the magical total depth percentage that a round, pear, marquise or oval must not exceed in order to remain a Class II (Class I and Class II diamond are well proportioned diamonds and Class III and Class IV are not). Once the total depth percentage exceeds 61%, it can be proven very easily by taking the tangent of the crown and pavilion angles and their corresponding crown heights and pavilion depths to show how light enters critical angles in the pavilion of the diamond and leaks out to create a fish eye in a round and deep bow-tie shadows in pears, marquise and ovals.

An avid reader of my book, “How to Buy a Diamond” pointed out that I did not send this point home well enough to my readers and visitors of my website. It is with his encouragement that I correct any omissions or explanations on this very point. With so many laboratories stating that total depths can exceed 61% my lack of emphasis on the importance of the “61% Factor” might have left too many question marks in the minds of some of the readers of my book, website and columns. The importance of the “61% Factor” can now be placed in the limelight that it rightfully deserves.

In closing, someone once asked me how important not going over the 61% really was in terms of total depth percentage? My response was short and to the point. Imagine that you are 61 steps from the edge of a cliff, how big a deal is that 62nd step?

by Fred Cuellar, author of the best-selling book “How to Buy a Diamond.” More questions? Ask the Diamond Guy
The founder and president of Diamond Cutters International, Fred Cuellar is one of the top diamond experts in the world, as well as a three-time Guinness Book record holder in jewelry design.
Fred Cuellar
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