Approximate varieties of rough in an average gem parcel
CHART C
The above percentages will vary from one mine
location to another. In some mines, one group or the other may have
a higher percentage.
Additional factors in the weight yield
of rough diamonds are its color and inclusions; both internal and external.
Inclusions, particularly, will affect the orientation of the rough
diamond during manufacturing and ultimately the final weight of the finished
diamond.
We
will now examine the previous ten varieties, and see how structure and shape
affect the percentage of the finished weight retention. The percentages discussed in the following
pages are very accurate and can be relied upon to consistently produce these
finished weights from the rough. (Whenever
there is a percentage range we will also discuss when to apply the lower or
higher percentage). It is recommended
that these percentages be used when purchasing rough in the field.
It should also be noted that we are
beginning to learn simple but efficient methods, which will later be combined
with other factors to be able to price the rough.
(c) Dodecahedrons
Dodecahedrons
and slightly modified dodecahedrons go through exactly the same process as
an octahedron. This group usually
composes 15% of a rough parcel. However,
because of the more rounded outline the weight yield is slightly higher. Weight retention of 51% to 55% can be obtained
from the rough. When making calculations
in the field we can use an average of 53%. The following two aerial view photographs,
photo 2.22 and 2.23, will reveal the difference in outline. Notice the slightly bulged outline in photo
2.23 of the dodecahedron as compared to the straight lines seen in photo 2.22.
diameter
of the rough is not proportional to its depth, the proportions of the finished
diamond will be poor and an excessive amount of weight can be lost. The weight retention can sometimes be as low
as 30%.
(Note: The above three categories account for 50%
in most rough gem quality parcels).
(e) Irregular dodecahedrons (whole
diamonds type 2)
Irregular
dodecahedrons are always elongated in outline (photo 2.5). The shape does not lend itself to be fashioned
into round brilliants but rather as a fancy cut. These diamonds are manufactured as one single
fancy shape diamond. To insist on
manufacturing into a round brilliant from an irregular dodecahedron; this
will result in excessive weight loss.
Notice in diagram 7.7A the widest diameter of the octahedron
becomes the sawed plane, which in turn, automatically gives us the table plane. The table plane is used as the foundation for
the entire layout of the remainder of the rough. Diagram 7.7B shows the point at which the saw plane will exist to
produce one smaller and one larger finished piece. Notice that the girdle plane of the larger finished diamond outline
is situated at the widest point of the rough. Also, the height of the sawing plane cannot exceed more than 16.2%
from the girdle plane of the larger section. If the rough is marked any higher,valuable rough material will be
wasted.
Given
the proportions that were used in diagram 7.7B the table size cannot be made
any smaller than 53%. Therefore, marking
the octahedron or dodecahedron above the 16.2% point will be taking away usable
material from the smaller section of the rough.
Fig. 8.4
Fig.
8.4 reveals a side view of a very shallow rough crystal.
Two possibilities exist. Either
brut the rough to reduce diameter compensating for the shallow depth, or stop
the bruting at a point in which the finished diamond will have shallow proportions. If the diameter of the diamond is left too
wide there will not be sufficient depth to complete the diamond to adequate
proportions. Very poor proportions
will be counter-productive to any extra weight that may be saved. The above examples are a key consideration
in deciding what possibilities exist in producing a finished diamond from
a whole rough crystal (non-sawables).
(b) The girdle plane on sawables and non-sawables
After
the rough diamond has been bruted and sufficient girdle thickness is made
to accommodate the desired crown height percentage, the diamond is marked
with a line around the girdle to give the diamond cutter a guide as to where
the crown and pavilion
Photo
10.11 is the manufactured rectangular shape rough that was seen in photo 10.10.
Now that
we have examined the shape of the rough diamond in relation to the various
possibilities for specific fancy shapes, we will now examine the approximate
percentage of rough fancy shapes that exist for each of the following groups.
