colour, presence of gas bubbles, internal cracks, and lack of definite orientation of the cut stones.
Structural lines.—Fine
structural lines are seen as the material appears to be formed of thin
layers prallel to the surface of the boule. These-lines may be seen
under the microscope in cut stones as well as when cut en cabo-chon.
The lines are curved, conforming to the shape of the boule. They should
be easily distinguished from the zonal distribution of colour in
natural stones. The stone should be examined under a microscope
immersed in a suitable liquid. Surface striations are also produced in
cutting and polishing and also lines due to repeated twinning on
natural stones, but they differ from structure lines of synthetic
material.
Gas bubbles.—Rounded
and elongated gas bubbles are also common in the boule. Cloudiness may
also be seen, which is considered as due to the collection of gas
bubbles in limited areas. It is really due to the faulty handling of
the furnace. Gas bubbles are characteristic of synthetic corundum but
not of natural corundum, which, however, may contain liquid inclusions;
which may also contain gas bubbles. Inclusions of hematite and rutile
may also be present in natural corundum. The presence of these
certainly indicate natural stones.
Internal cracks.—The
boules are under great strain and should be handled carefully. But when
once the strain is relieved these synthetic stones will not fracture
further while being sawed, cut or polished, but due to their
brittle-ness internal cracks may develop. When there are large internal
cracks visible easily, they are known as feathers.
Orientation of cut stones.—Both
synthetic and natural stones are identical as regards composition and
physical properties. As the coloured stones show pleochroism, to obtain
the deepest colour and the best advantage the table should be cut
parallel to the basal pinacoid. This could be best done in natural
stones, but not in synthetic stones as
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