a round stone, the depth of a stone, measured as a percentage of the
diameter, is a direct indication of the spread of that stone.
Basically, one could say that a higher depth means a lower spread,
while a lower depth indicates a higher spread.
using well-known and tested procedures for rounds on fancies does not
always work, and very often, one could be misled by simply using the
rule for rounds on fancies. The depth of a princess-cut is a classic
In this, both technical as psychological differences are at play.
1. The technical difference
round brilliant is a symmetrical stone with one main pavilion angle and
one main crown angle. As such the depth of a stone is a direct result
of that pavilion angle, that crown angle in combination with the table
size and the thickness of the girdle.
Because of the round brilliant being such a straightforward stone, the rule that depth relates to spread can be applied.
so in a princess-cut however. To start, a princess-cut has two main
pavilion angles and two main crown angles. On top of that, the table
size of a princess varies considerably from 55% up to higher than 80%.
Below, you can find an illustration of A.G.S.-Laboratories, which
clearly shows the different angles.
you should pay attention to the pavilion of this stone. The ‘pavilion
2’ goes from the girdle to the culet, and it is therefore this pavilion
angle, which sets the pavilion depth.
‘Pavilion 1’ on the
other hand can vary considerably in angle, without the pavilion depth
being affected. It is precisely in this area that a cutter can hide a
lot of weight without the total depth being affected.
2. The psychological difference
looking at a round brilliant, we are used to seeing total depths around
60%. Psychologically, a total depth of 70% and higher in a princess-cut
automatically seems extremely deep.
However, one should consider that both depths are not measured in the same way.
a round, the total depth is a percentage of the average diameter of the
stone. In a princess however, and in any fancy shape, for that matter,
the total depth is a percentage of the lowest diameter.
consider a perfectly square princess with a diameter of 6mm. from side
to side. With a depth of 4.5 mm., we will calculate a depth percentage
However, if we would measure the diameter from point
to point, the diameter would be 8.49 mm., and the depth percentage of
the highest diameter is only 53.03%.
Now, if we would work
with total depth compared to the average diameter of a princess-cut,
like we do in rounds, the depth percentage is 62.13%.
agree with me that there is a psychological difference between a depth
percentage of 75% (traditional measurement) and 62.13% (measured on
is a very short, but important article. It shows how even the simplest
rules of rounds do not necessarily apply to fancy shapes.
I hope that you have enjoyed this little arithmetic.