s
is well known, the rough material is formed in boules or pear-shaped
drops under an inverted blowpipe. The powdered material is fed in with
one of the gases and passes through the flame, melting as it goes, and
then accumulating and crystallizing below as a boule. The top or head
of this boule is rounding from the start, and hence the successive
layers of material gather in thin curved zones. The color and structure
of these successive zones are not perfectly uniform, hence when cut
stones are made from the boules these _curving_ parallel layers may be
seen within by the use of a good lens, especially if the cut stone is
held in a strong crossing light, as was suggested when directions were
given above as to the best way to look for "silk" in a natural stone.

Owing to the shape of a well cut stone it is sometimes difficult to get
light through the material, yet by turning the stone repeatedly, some
position will be found in which the curving parallel striae can be seen.
They are easily seen in scientific ruby, less easily in dark blue
sapphire, but still they can be found on close search. In the light
colored stones and in white sapphire, the difficulty is greater, as
there are no color variations in the latter case. However, the value of
white sapphire is so slight, whether natural or artificial, that it is a
matter of but little moment, and what has already been said as to
natural defects, applies to white sapphire as well as to the colored
varieties, and absolutely clear and perfect natural white sapphire is
rare.

One more distinguishing mark of the scientific stones may be added to
give full measure to the scheme of separation, that no one need be
deceived.

The surface finish of the scientific stones is rarely as good as that of
the natural material and it appears to be more difficult to produce a
good polish on scientific stones than on natural ones. The degree of
hardness of the scientific stones seems to be slightly variable in
different parts of the same piece so that the polishing material removes
the surface material unequally, leaving minute streaky marks on the
surfaces of the facets. Possibly this condition might be remedied by
skillful treatment, but hardly at the price obtainable for the product,
so that a close study of the surface finish will sometimes help in
distinguishing between natural and artificial material. Any fine
specimen of natural ruby or sapphire will have usually received very
ex