An octahedron crystal is sawn in half, so
the crown of the larger stone is already in place, with a smaller stone
polished from the remainder (see image 4, above). If the octahedron has all its
corners intact, then two princess cuts can be polished, with the yield as high
as 80 per cent. So a one-carat rough can yield as much as two 0.40-carat princess
cuts with a total weight of 0.80-carats.
This explains why one-carat, square,
fancy-shape diamonds cost around 30 per cent less than rounds, which typically
have total yields of only 45 to 50 per cent.
If the octahedron has a few damaged points
or inclusions in that area, then it is common to polish one or more diamonds
with “cut corners”, like a radiant cut.
The octahedron is a common crystal habit
for diamond. This shape is called a “sawable” because the stone is usually sawn
into two stones, as opposed to a “makeable”, (a rough diamond that will be
polished into a single stone). It can be sawn right on the edge into two equal
halves, with the saw plane becoming the table of each diamond. (See image 4.)
Sometimes the saw blade is run just above
half way, leaving the thickness of the crown height to make one diamond a
little heavier.
As shown in the images of a rough diamond,
this also helps explain why many cuts, and princess cuts in particular, tend to
have very large tables. In fact, princess-cut tables are usually too large by 5
to 20 per cent more than the optimum. Sometimes the crown height is so low that
it reduces optical performance.
The first square (and rectangular) outline
diamonds to be cut were the step-cut family, where the facets run parallel to
the outside edge. These include Carré (small square cuts), bagutte and emerald
cuts. The next article in this series will cover that family.
The radiant cut article, featured in
November 2008, discussed how the radiant cut (first patented in 1976) is one of
the more recent diamond cuts and was the first rectangular or square cut to
have a complete brilliant facet pattern applied to both the crown and pavilion.
This cut presented more sparkle than the emerald step cut.
The princess cut article (March 2008)
mentioned how this cut was designed as a square version of the round-brilliant
cut, also known as a square modified-brilliant. The goal of its development was
to achieve the weight retention of emerald step cuts when cutting from
octahedral crystal rough, and the superior scintillation and -brilliance of the
round-brilliant cut.
These are a princess cut’s selling points –
a bigger carat weight, for less money and plenty of sparkle.
The Regent is a square, proprietary, hearts
and arrows cut designed to show the same hearts and arrows patterns as round
brilliant cuts (see images 2 and 3).
It is important to remember that the eight
faces of an octahedra (four crystal plane directions because the opposite sides
are parallel) are also the easiest cleavage direction and as such, all square
cuts are prone to chipping from the girdle edges during setting. For that
reason it is a good idea to suggest rings to clients that are insured. Slightly
thick to thick girdles are less likely to chip. Chipping can also become an
issue with sharp corners – 90 degree corners – such as the princess cut.
Buyers should also be aware of Fish-Eyes
(girdle reflections). It is a good idea to have a comparison stone to compare
these stones for brilliance, fire and scintillation.