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Diamond 2
Distinctive features Hardness
can be an important factor in distinguishing diamond from
other stones. It is in fact the only gemstone capable of scratching
corundum. The best modern imitation, cubic zirconia, is less
hard than corundum, so the two can easily be distinguished
by comparing them with corundum, although the results of the
tests must be observed under a binocular microscope or at
least a lens. Diamond's exceptional hardness is also displayed
by the facet edges, which are sharper than in imitations.
This is best appreciated in relation to zircon, which has
brittle, easily damaged edges, and the less hard imitations
such as synthetic rutile and strontium titanate. If the stones
are turned between the thumb and forefinger, the two softer
substitutes feel more slippery, almost oily, compared with
diamond, because of this difference in the edges.
Another characteristic can be seen in the girdle,
but only when it is not faceted. Due to the procedure used
in turning brilliant cuts, the girdle of a diamond will have
a satiny finish, similar to that of finely ground glass (in
recent years a similar effect has been achieved with cubic
zirconia, but no other imitation displays this). Also, when
there are numerous flaws like minute cracks extending from
either side of the girdle (these are known as "bearding"
and are due to inexpert turning of the rough stone), it is
bound to be diamond. A brilliant cut can display small facets
on the girdle or extending from the girdle toward the pavilion
(or more rarely, toward the crown).
These are the remains of the outer surface of
the uncut octahedron (nearly always with minute, crystallographically
oriented shallow triangular cavities), or of the faces of
octahedral cleavage. In the latter case, small steps can be
observed between contiguous, specular plane surfaces. However,
these details are only readily visible if magnified at least
10-20 times. Sometimes small triangular facets alone are seen
extending from the girdle. Called extra facets or supplementary
facets, these are produced by polishing of the facets just
described or by the elimination of some small, almost superficial
flaw. Although very similar to the foregoing and not usually
found on imitations, these facets are less distinctive in
that they could be produced on any other stone.
Other distinctive features are related to the
fact that the most frequently used, brilliant and emerald
cuts, are designed to make the most of the high refractive
index of diamond and obtain the maximum possible total internal
reflection of the light coming from the table facet. Therefore
if a diamond is placed with the table facet against tiny written
characters, nothing will be visible through it, unless the
pavilion is extremely flat. With imitation diamonds of a much
lower refractive index, such as YAG (Yttrium Aluminum Garnet),
something will be visible through the stone and still more
will be seen through synthetic spinel and colorless sapphire.
The difference is more obvious with emerald cuts than with
brilliants. A similar effect, but confined to brilliant-cut
stones, can be seen through the table facet.
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