stone,
being placed on the surface of the mill, and the opposite end of the
stick to which it is cemented being inserted in one of the holes of
the gauge, the mill is put in motion by turning a winch, and the stone
kept steady on it.
When
the stone has all the facets, the cutting mill is taken out and
replaced by one of brass, on which the polishing is performed by means
of fine emery and rotten-stone, in the same manner as before. A good
judgment is required in determining the form and proportions best
adapted to set off any particular stone to the best advantage. If the
color is full and rich, its transparency perfect, and its refractive
power considerable, the best form to give it is the brilliant. If, on
the contrary, the color is dilute, the most advantageous method of
cutting it is, to cut the table side (pavilion) brilliant fashion, and
the collet side (culasse) in steps; by this means the table itself will
be left dark, while all the light reflected from the steps on the under
side of the stone will be thrown up into the facets, by which the table
is surrounded. The French lapidaries cut the most perfect sapphires in
a square or octagon form, with a single delicate step between the table
and the girdle, and three or four steps between the girdle and the
collet.
If
the sapphires possess a varying chatoyant lustre, or are of a small
size, their form is always hemispherical or elliptical, without any
flat facets; the flatter the ellipse the more the varying lustre is
diffused over the surface of the stone ; whereas with a high ellipse it
is condensed on a single spot.
In
setting sapphires we always use foil answering to their color. The ruby
is set with a reddish gold foil, or a foil of copper or red glass; the
blue sapphire with a silver foil, or blue-colored foil, or with
feathers of blue ducks, pigeons,
