y or sapphire such lines will be few and
difficult to find, but in some position or other they will usually be
found if the search is even as careful as that which one would
habitually employ in looking for defects in a diamond. In the vast
majority of cases no such careful search will be required to locate
"silk" in natural rubies, and if a stone that is apparently a ruby is
free from such defects it is almost a foregone conclusion it is a
scientific stone.

Another common type of defect in corundum gems is the occurrence of
patches of milky cloudiness within the material. A little actual
acquaintance with the appearance of this sort of defect in natural
stones will make it easy to distinguish from the occasional cloudiness
found in scientific stones, which latter cloudiness is due to the
presence of swarms of minute gas bubbles. These tiny bubbles can be seen
under a high power lens, and this suggests a third feature that may be
used to tell whether one has a natural stone or not.

Natural rubies and sapphires, like scientific ones, frequently contain
bubbles, but these are always _angular_ in the natural stones, while
those of the scientific stones are generally _round_ or rounding, never
angular.

To sum up the suggestions already presented it may be said that, since
natural and scientific corundum gems are composed of essentially the
same material, and have identically the same physical and chemical
properties, and frequently very closely resemble each other in color, it
is necessary to have recourse to some other means of distinguishing
between them. The best and simplest means for those who are acquainted
with the structural defects common to natural corundum gems is to seek
for such defects in any specimen that is in question, and if no such
defects can be found, to be very sceptical as to the naturalness of the
specimen, inasmuch as perfect corundum gems are very rare in nature, and
when of fine color command exceedingly high prices. No jeweler can
afford to risk his reputation for knowledge and for integrity by selling
as a natural stone any gem which does not possess the minor defects
common to practically all corundum gems.

STRUCTURAL DEFECTS OF SCIENTIFIC STONES. So far our tests have been
mostly negative. It was said, however, that spherical bubbles sometimes
appear in scientific gems. Another characteristic _structural_ defect of
practically every scientific gem may be utilized to distinguish them. A