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Diamond 5
The influence of purity Many
diamonds contain crystalline inclusions or discontinuities
(even just fractures) which reduce their transparency, by
interrupting the paths of the light rays. Stones free of inclusions
or visible discontinuities are described as pure, and value
diminishes as visible imperfections increase.
Classification is based on visual criteria and
detailed diagrams are available to establish a basis of comparison.
Stones are conventionally examined under a 10x lens. The grades
of the scale of purity are shown in Fig. E and are referred
to by internationally accepted sets of initials, according
to the English terminology. Pure stones of IF grade are uncommon;
VVS and VS grades are of a good level of purity, SI and PI
grades are not so pure and PZ and P3 are the lowest categories.
Naturally, stones combining perfect purity with excellent
color are rare, especially those weighing several carats.
If one is prepared to compromise over a single characteristic
such as color, it will be easier to find stones of good purity
and weight. Similarly, it will be easier to find a stone of
good color and weight but mediocre purity; and those of middling
color, medium purity, and small size are obviously the most
common.
The above should give some idea of the complexity
and also the degree of organization of criteria used for the
valuation of diamonds. In addition to these fundamental criteria
there are others based on the proportions of the cut (when
not ideal, they reduce the stone's powers of light reflection
and refraction) and surface finish, which are taken into account
in establishing value. Proportion and finish are judged as
very good, good, medium, or poor.' Sometimes these judgments
are based on measurement, but usually they are established
by the visual evaluation of experts.
Simulants and synthetics Diamond
has been imitated by glass and special glass with a mirror
backing to increase reflection, called strass. It has been
imitated by colorless, synthetic spinel and corundum ever
since these began to be produced. Over the last few decades,
moreover, it has been imitated by a whole range of synthetic
and artificial products:
• strontium titanate or fabulite, which
has much higher dispersion than diamond, with striking iridescence,
low hardness, considerable brittleness and density;
• synthetic rutile or titania, which
has very high dispersion with striking iridescence, marked
birefringence, high density, and low hardness;
• YAG (Yttrium Aluminium Garnet), which
has a slightly lower refractive index than diamond, a hardness
of less than 9, and greater density;
• GGG (Gadolinium Gallium Garnet),
which has a fairly low hardness and double the density of
diamond;
• cubic zirconia (cubic zirconium oxide),
djevelite or phianite, the most recent and hardest to distinguish
from diamond.Its density, however, is much greater, and its
hardness is less than 9.
Many other artificial substances are similar
to diamond, but are not used as imitations because their cost
is generally much higher and their characteristics no better
than those simulants already mentioned. Diamond has been and
still is manufactured synthetically, in minute crystals, only
suitable for industrial purposes. It has also been produced
experimentally for use as a gemstone and a few gems of about
0.25-0.45 carats have been cut. But it is so hard to make
the crystals grow that the cost would be much higher than
that of natural diamonds obtained from known deposits.
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