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Diamonds: The 61% Factor
(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®
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