The procedures listed herein for gem cutting do not apply to all
gemstones. Stones that are especially brittle, soft, or difficult to
polish require additional procedures or special techniques. Many
lapidaries may deviate from these procedures. Some of the steps of
cutting and polishing are merely matters of personal opinion and vary
somewhat from cutter to cutter. There are several detailed texts on the
art of gem cutting; the descriptions herein are designed to give the
reader only a general idea of the procedures and techniques involved.
The cutting and polishing of cabochons require several steps. The
initial step is sawing. Assuming that the rough gem material is large
enough to be sawed (larger than about half an inch in diameter), it is
clamped into the carriage of a diamond saw (fig. 3) and cut into slices
about ⅜-inch thick. The blade of the saw is mild steel that has been
impregnated with diamond dust around the edge, hence the name diamond
saw. The blade is rotated rapidly, and the material to be cut is “fed”
to the blade by a sliding carriage on which the gem material is clamped.
The extreme hardness of the diamond dust in the edge of the blade
enables the saw to cut through several inches of gem material in a few
minutes. The lower portion of the saw blade is immersed in a mixture of
kerosene and oil, and the rotating saw blade carries with it some of the
kerosene-oil mixture; this acts as a coolant and lubricant for both the
saw blade and the material being cut. Without this lubricant, the heat
generated by sawing would shatter most gem materials and also damage the
saw blade. As this “slicing” or sawing of the material usually takes
several minutes, a weight and pulley are generally used to give the gem
material the necessary pressure against the saw blade. When cut through,
the “slab” of gem material falls into the kerosene-oil mixture at the
bottom of the saw or onto a special platform that cushions its fall.
[Illustration: Fig. 3. Diamond saw.]
Motor
Clamp
Diamond-charged blade
Carriage
Stone
Weight
After being sawed, the slab of gem material is examined, and the
location and size of the stones to be cut from the slab are determined.
The desired outline of the shape of the gem to be cut is marked on the
slab with a pointed piece of aluminum rod; ordinary pencil marks are not
used because they wear away too quickly in the cutting process. Once the
area from which the gem is to be cut has been selected and the outline
of the gemstone has been marked on the slab, the excess material is
trimmed away by a smaller diamond saw known as a trim-saw. In some slabs
the excess material can be broken and “nibbled” away with a strong pair
of pliers.
The remaining portion of the stone is usually held by hand and ground to
the desired shape using the previously scribed mark as a guide. This is
done using a relatively coarse-grained (about 150 grit) specially made
carborundum grinding wheel.
Now that the desired outline has been obtained, the stone is firmly
affixed to a slender wooden or hollow aluminum dop-stick (fig. 4). The
process whereby the stone is attached to the dop-stick with a specially
compounded jeweler’s wax is called dopping. The dop-wax is heated over
an alcohol lamp or candle flame until it is soft and pliable and is then
spread around on the end of the dop-stick and formed into a mass about
the right size and shape to fit the back of the gemstone. The stone is
likewise heated, and the wax is applied to the back of the stone while
both wax and stone are hot. Upon cooling, the wax firmly fixes the stone
to the dop-stick. The dop-stick allows the lapidary to have firm control
of the stone during all later stages of cutting and polishing.
[Illustration: Fig. 4. Cabochon properly attached to dop-stick.]
CABOCHON
DOP-WAX
DOP-STICK
The top of the dopped gemstone is worked against the coarse carborundum
grinding wheel until it is a rough approximation of the desired shape.
The stone is then worked against a much finer-grained (about 220 grit)
grinding wheel to remove the irregularities left by the coarse grinding
and to further smooth and shape the surface of the gemstone. At all
times while grinding, a small flow of water should be directed on the
grinding wheel to keep the stone cool. Grinding on the stone for even a
few minutes without cooling may result in the shattering of the gemstone
because of heat created by friction of the stone against the grinding
wheel. If the lapidary keeps the surface of the grinding wheel wet,
there is little chance of damaging most gem materials.
The next phase of cabochon cutting and polishing is sanding. The
gemstone is worked against two sanding drums of different grit size.
This sanding can be done with the sandpaper surface either wet or dry,
as needed or as preferred by the lapidary. However, great care should be
exercised during sanding so that the stone is not overheated.
Overheating can easily occur whether the sandpaper is used wet or dry.
As in grinding, sanding is first done on coarser grit paper (about 300
grit) and last on finer paper (about 600 grit). It is in the sanding
process that the first hint of polish is noted on the surface of the
stone. After sanding, the gemstone should have perfect form with no
surface irregularities, a very finely textured surface, and only very
minor scratches left from sanding. The gemstone is now ready to be
polished.
[Illustration: Fig. 5. Cabochons at various stages of cutting and
polishing. Left to right: trimmed from slab: ground to outline; after
rough grinding; after sanding; polished.]
At this point the procedure depends on the nature of the gemstone being
polished. Most gem materials are worked against a buffing wheel that is
impregnated or saturated with a mixture of some polishing compound and
water. A soft felt buffing wheel with cerium oxide as the polishing
agent is used for many materials. The mixture of cerium oxide and water
is usually applied to the buffing wheel with a small brush. The lapidary
should once more be careful not to overheat the stone. If the stone
becomes too hot to hold to the underside of the cutter’s wrist, it
should be permitted to cool for a few seconds before continuing. After
polishing on the buffing wheel, the gemstone should have a fine, high
polish and be free of any scratches or surface irregularities. The
finished gemstone is removed from the dop-stick by heating the dop-wax
and pulling the stone loose. Any excess wax that hardens again before it
can be removed from the stone by hand can be dissolved away by rubbing
with an acetone-soaked cloth. Figure 5 illustrates the desired
appearance of the gemstone at the end of each of the steps of cutting
and polishing.