The principles involved in faceting are about the same as those in the
cutting of cabochons, but the equipment and technique are considerably
different. The equipment required for the facet cutting of gemstones is
built into or attached to a small specially constructed table (fig. 7),
and the unit is commonly called a facet table. Most faceted gemstones
are cut to obtain the largest flawless stone possible from the rough
material. Therefore, one of the first and most important steps for the
lapidary is to decide how the stone is to be cut from the rough crystal
or pebble. The colors that can be obtained from the gemstone must also
be considered, and the cutting of the stone oriented so that its best
color is displayed. The lapidary also selects the orientation of the
stone in relation to the cleavage or cleavages. It is difficult or
impossible to polish facets of gemstones that are cut parallel to a good
cleavage direction.
[Illustration: Fig. 6. Nomenclature of the standard American brilliant
cut.]
TOP VIEW
SIDE VIEW
Star facet
Crown main facet
Crown girdle facet
Pavilion girdle facet
Pavilion main facet
TABLE
CROWN GIRDLE
PAVILION
CULET
BOTTOM VIEW
Once the orientation of the gemstone to be cut from the rough material
has been determined, the stone is dopped onto a special metal dop-stick
that fits into the chuck of the facet head. The chuck is tightened so
that the position of the stone on the end of the arm of the facet head
is firmly fixed, and the facet head is adjusted so that the first facet
that is cut is the horizontal, top facet of the stone or table facet
(fig. 6). The table facet is cut by grinding the gemstone on a flat
cutting lap that is diamond impregnated (fig. 8). By minor adjustments
of the facet head, the lapidary can precisely control the location of
the table facet. As soon as the table facet has been ground to the
proper size, the cutting lap is removed from the lap plate, and the
polishing lap is secured in place. Many different kinds of polishing
laps and polishing compounds may be used depending on the properties of
the material being polished. However, one lap and one polishing compound
are usually sufficient for each gem variety. After the polishing lap is
secured to the lap plate, the lapidary adjusts the facet head so that
the stone is in exactly the same position relative to the lap that it
was during the cutting of the table facet. The polishing lap is run wet
or damp with water, as is the cutting lap, and small amounts of the
polishing compound are applied to the surface of the lap while the facet
is being polished. The minor scratches left by the cutting process are
gradually removed, and a fine lustrous polish develops on the facet. It
is especially important to take care in achieving a perfect polish on
the table facet, as this facet occupies a large area of the crown of the
gemstone. When the cutting and polishing of the table facet are
completed, the gemstone is still rough or uncut in all portions except
for this single, large, polished surface.
[Illustration: Fig. 7. Facet table.]
Water
Light
Adjusting ring
Post
Arm
Chuck
Stone
Abrasives
DIAMOND DUST
CALCIUM OXIDE
LANDE-A
[Illustration: Fig. 8. Grinding the table facet on a rough stone.]
CHUCK
DOP-STICK
DOP-WAX
STONE
LAP
The gemstone is then removed from the dop-stick by melting the dop-wax
and is dopped once more so that the plane of the polished table facet is
perpendicular to the axis of the chuck and arm of the facet head (fig.
9). Great care should be taken by the lapidary to insure that the table
of the stone is exactly perpendicular to this axis, or the proper
placing of the later facets on the stone may become very difficult.
[Illustration: Fig. 9. Stone dopped to table facet.]
TABLE FACET
DOP-WAX
STONE
DOP-STICK
Once the stone has been properly dopped to the table facet, the lapidary
is ready to proceed with the cutting of the outline of the stone. If it
is to be a brilliant cut, the stone is ground perfectly round in
outline; if it is to be an emerald or step cut, it is shaped so that it
is square or rectangular in outline. This process is called preforming.
The arm of the facet head is lowered on the post until it is horizontal,
and the stone is worked against the cutting lap until the desired shape
is obtained. When the preforming process is completed, the stone should
have the desired outline of the finished gem (fig. 10).
[Illustration: Fig. 10. Preformed stone dopped to table facet.]
DOP-WAX
STONE
DOP-STICK
The lapidary is now ready to proceed with the cutting of the pavilion of
the stone. The arm of the facet head is raised to the proper angle for
cutting the main pavilion facets. The angle at which the main facets are
cut is very critical in determining the beauty of the finished stone.
The required angle at which these facets must be cut varies with the
refractive indices of the different varieties of gem minerals. If the
facets are not cut at exactly the proper angle, light entering the top
or crown of the gemstone can pass completely through the stone, instead
of being reflected back out of the crown facets. The result is a dull,
lifeless stone that appears to have a “hole” or “fish-eye” in the
center. Stones that are cut in this manner are greatly reduced in value.
The angle at which the facets are cut is controlled by the adjustment of
the height of the arm of the facet head on the post. The lapidary will
continually adjust this height, because the angle between the arm and
the surface of the lap changes slightly as the facet is ground down to
its proper place and size.
[Illustration: Fig. 11. Proper sequence of cutting of the pavilion
facets. Left to right: four main facets; all eight main facets; half of
the pavilion girdle facets; completed pavilion.]
The standard American brilliant cut will be used as an example of facet
cutting. Procedure for all other cuts is essentially the same to this
point. After the eight main pavilion facets have been cut, the cutting
angle is changed a few degrees, the arm of the facet head rotated
slightly, and the sixteen pavilion girdle facets or “skill” facets, as
they are often called, are cut (fig. 11). The pavilion girdle facets
should meet exactly in the center of the main facets at the girdle of
the stone. The pavilion girdle facets should neither overlap, nor should
there be any space between them (fig. 12). After the pavilion girdle
facets are cut, the cutting of the pavilion of the gemstone is
completed. The facets are then polished on the polishing lap at the same
angles and in the same order as they were cut, and the pavilion of the
gem is completely finished.
The stone is then removed from the dop-stick by melting the dop-wax and
is re-dopped to the pavilion facets so that the crown of the stone is
now exposed for cutting. Before the lapidary proceeds with the cutting
of the crown, it is necessary that the stone be perfectly centered on
the dop-stick and that the plane of the table facet be perpendicular to
the dop-stick and to the axis of the arm of the facet head. The eight
main facets are cut first, with numerous adjustments being made by the
lapidary to insure that the proper angle is maintained (fig. 13). Then
the cutting angle is changed a few degrees, the arm of the facet head
rotated slightly, and the crown girdle facets are cut. The crown girdle
facets are placed very similarly to the pavilion girdle facets except
that they are shorter. The crown girdle facets should be joined in
exactly the same way as the pavilion girdle facets. When these facets
are properly cut, the cutting angle is again changed, the arm rotated,
and the eight star facets are cut. This completes the cutting of the
crown of the stone. The cutting lap is removed from the lap plate, and
the polishing lap is secured into place. The facets are carefully
polished in the same order that they were cut. After the last star facet
has been polished, the stone is removed from the dop-stick. Any excess
dop-wax is removed from the stone by means of a solvent, and the full
beauty of the finished gem is revealed.