Cut is by far the most
difficult to measure of the 4 Cs. In an ideally-proportioned diamond, all the light
entering the diamond from the top should bounce within
the diamond and reflect back through the top, giving
the stone maximum brilliance and fire. If the stone
is too shallow or too
deep, some light will escape through
the bottom of the diamond, giving the appearance
of shadows when viewed from the top.
It's easy to see that the deep-cut
diamond shown above may have a
higher carat weight, but it is clearly the less desirable
stone! Many jewelers will not discuss cut proportions
unless the customer specifically asks. A stone rich
in carat weight but poorly proportioned can be deeply
"discounted," giving the buyer a false impression
of a great deal.
Ideal Proportion Metrics. In order to assess how well a given
diamond is cut to ideal proportions, you will have to
measure the diamond. If the stone has a
GIA, AGS, or other certificate, the measurements will be on the
certificate. Note that it is common to use the relative
measurements of the diamond, expressed in percentages,
rather than the absolute measurements which are typically
in millimeters (mm).
In 1919, Dr. Marcel Tolkowsky empirically
calculated the ideal proportions of a round diamond
as part of his Ph.D. thesis in Mathematics. According to Tolkowsky,
an "ideal cut" has the following
Round in shape, and brilliant-cut
Depth percentage: 59%
Table percentage: 53%
Crown height percentage: 16%
Pavilion depth percentage: 43%
Girdle thickness: Medium and even all the way
around the diamond
Perfectly aligned and formed facets
Very small or absent culet
It should be obvious that finding a Tolkowsky diamond
is an expensive and time-consuming task. To further complicate
matters, recent GIA studies demonstrate that, in contrast with
Tolkowsky's 53% specification, the brilliance of a diamond varies
significantly with many other specifications beyond the table.
Let your eyes decide. While the debate continues over the
question of the "ideal" specifications, our suggestion is that you
use the above numbers as guidelines only, and let your eyes be the
final judge. This is why it's critical to see and feel the diamond
you purchase, at a reputable local jeweler.
Other Factors Related to Cut. Aside from the right overall proportions
and a shape that suits the owner's personal taste, a diamond's value
and intrinsic beauty are governed by the
workmanship of the diamond cutter. Diamond cutter workmanship
includes facets, girdle, cutlet, symmetry, and finish.
Facets. Facets are the flat polished surfaces of a gemstone.
When looking at diamonds under a microscope,
compare each facet with the ideal form presented
here. For example, a round diamond has exactly 58 facets
-- 33 above the girdle, and 25 below.
When looking at actual
stones through your local jeweler's microscope, refer
to the diagram above and try to identify and rate each
of the facets. Extra, absent, or deformed facets decrease
the value of the stone.
Girdle. The girdle of a diamond is the middle,
or fattest part, of the diamond, measured from
"extremely thin" to "extremely thick":
Thin girdles are prone
to chipping. Extremely thick girdles hide a lot of weight,
so you end up paying for a heavier diamond which doesn't
look that big. The ideal is an even, medium girdle.
This applies to all shapes EXCEPT for the tip of the
pear shape, in which a thick girdle is preferred to
make it less prone to damage. When examining the girdle,
make sure it is well-faceted and polished. Some diamond
cutters will omit this step to save time
and to avoid removing weight from the stone.
Culet. The culet is the bottom part of the
diamond, where the pavilion comes together in a point
(see the diagram above, in "facets" section).
Culets are rated as "none", "small",
"medium", and "large." The ideal
is "none," but if an otherwise perfect stone
has a medium or large culet, it may still be a worthwhile
Symmetry. Check the following aspects of the
stone to make sure it is completely symmetrical:
- Make sure the culet is precisely in
the middle of the stone when viewing it from the bottom.
- Verify that
the pavilion and crown have the same angle
all the way around the stone, and are not bowed out
- If you're
purchasing a heart shape or any other fancy
shape, be sure there are no deformities. For example,
some heart-shaped diamonds will have unsightly asymmetrical
lobes. Some princess shape or radiants are not quite
- Inspect the table to make sure it is flat and symmetrical.
Finish. There's no hard and fast rule to judge
the finish of a stone. However, the
more a diamond cutter polishes the stone, the more carat
weight (and value) it loses. The cutter's
goal will be to polish the stone just enough to satisfy the potential
customer. Polishing also removes surface flaws
related to clarity, so an "IF" diamond
could potentially be turned into an "FL" simply by polishing it.