Thu, 17 Nov 2005

Scallops

Index

Figure
1 shows the typical case where the break facets are equally-spaced
around the circle (360°/16 = 22.5° apart). Their slopes are shown for
the main slopes and proportions cited. They are 0.86% and 0.83% deep,
respectively, for the pavilion and crown = total of 1.69%.

With
a girdle thickness of 1.0% at the thinnest points (midpoints) this
gives a thickness of 2.7% at the thickest points (endpoints). Some
gemologists report one, some report the other; if they report the
thinner one you must add 1.7% to the depth of the pavilion, crown, and
girdle to get the correct total depth of the stone.

Index

To
cut the facets at accurately-spaced angles around the stone (azimuths),
most faceting machines use an ‘index wheel’ (called a ‘click wheel’ by
most diamond cutters). This is a notched wheel, like a gear, into which
a ‘latch’ falls to lock the rotary position of each facet.

Many
colored-stone cutters use a variety of index wheels with different
numbers of equally-spaced notches. Figure 2 shows the spacings of
common wheels = 32, 48, 64, 72, 80, 96, or 120 notches. With a 32-tooth
wheel the mains are cut at 4,8,12,16,20,24,28,32 and the stars at
2,6,10,14,18,22,26,30. Usually the break facets are cut halfway between
the mains and stars – at 1,3,5,7,9,11,13,15,17,19,21,23,25,27,29,31.

This photo shows an 80-notch index wheel on a Raytech-Shaw faceting machine. This number is used for special cuts

This
photo shows a click wheel from a diamond-cutting rig. It has only those
notches needed by the brillianteerer for break and star facets.

Photo courtesy of Garry Holloway, 2005 May

 

Offset Scallops

Figure
3 shows what happens if the scallops are cut at various indexes shown
in Fig.2, keeping the same minimum thickness of 1.0% between the
pavilion and crown scallops. Note the original example of 0.86% and
0.83%, with equal thickness at both ends, shown in black, when the
indexes are equally-spaced and halfway between the mains and stars.

Some
cutters do this twisting of break facets intentionally for reasons they
deem important. More recently, GIA and AGS have written on this subject
as to how it affects the appearance and weight of a diamond.

If
the index is closer to the main than to the star index, as shown above
the black example, the thickness decreases at the tips of the mains and
increases at the join of the break facets.

If it is closer to
the star index (at the join of the breaks) than to the main index, as
shown below the black example, the reverse is true.

This
discussion has been limited to index wheels with a full set of
evenly-spaced notches, which is typical of general-purpose faceting
machines. It was intended to show that the intentional twisting seen in
some diamonds is less than can be provided by such wheels.

For
diamond-cutting, click wheels are made with only those teeth necessary
and can be made with any spacing the cutter requests from a click-wheel
manufacturer.

Figure 4 is of special interest to reduce depth
of scallops. On an 8-pointed round brilliant cut with Tolkowsky’s main
slopes and standard indexes, as in Fig.1, the combined depth of
pavilion and crown scallops is 1.69%. By adding another set of break
facets between these, the combined depth of pavilion and crown scallops
is reduced to 0.75%. The slopes of the girdle facets are different from
Fig.1.

Figure
4b shows an interesting variation; the pavilion and crown are rotated
so that the middle break facet of one aligns with the main facet of the
other. In this way the thickness variation of the girdle is almost
eliminated.

Bruce L. Harding 2004 Nov 16