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Diamond 3
By steadily tilting a stone of lower refractive
index than diamond, and looking through the table, a nonreflecting
transparent triangle can be seen to appear in the pavilion,
with its apex at the center and its base toward the edge of
the table opposite the observer. The lower
the refractive index of the stone, the smaller the angle at
which this will appear. In diamonds, this effect is very difficult
to see, except in poorly cut stones with very shallow pavilions.
Single refraction is another characteristic
that distinguishes diamond from zircon, which is strongly
birefringent and from an infrequent imitation of diamond:
synthetic rutile, which is even more strongly birefringent.
The famous dispersion in diamond, although considerable, is
much less than that of synthetic rutile and strontium titanate;
but these now uncommon imitations look positively iridescent
when viewed through a lens and even to some extent with the
naked eye.
Given the constancy of shape and proportions,
at least within certain limits, of stones with a round brilliant
cut, a given weight can be said to correspond to a given diameter.
If the diameter of a stone presumed to be a diamond can be
measured with some precision, one can check to see whether
it has a suitable weight (in which case it will either be
diamond or a stone of comparable specific gravity), or whether
the weight immediately rules out the possibility of its being
diamond.
Generally simulants are too heavy, as in the
case of cubic zirconia, GGG (Gadolinium Gallium Garnet) or,
to a much less obvious extent, zircon or YAG (Fig. A.). This
method could not be used to distinguish diamond either from
synthetic spinel, colorless topaz, or colorless sapphire,
as their relative densities are too similar, although all
of them have other characteristics unlikely to deceive any
but the most casual observer.
It was mentioned in discussing physical properties, that the
very high refractive index of diamond a outside the range
of normal refractometers and the same can be said of many
of its imitations. In compensation, however, diamond has other
characteristics, such as reflectivity and thermal conductivity,
which are quite different (because much higher) than those
of its present substitutes. Small instruments the size of
a pocket calculator have been produced to measure these characteristics,
making a rapid distinction possible.
Occurrence Gem quality diamonds are found in
about twenty different countries, a dozen of which are in
Africa, three in Asia, one in Oceania, and three in South
America. By far the largest producer is South Africa, including
the neighboring Lesotho, Botswana, and Namibia. Next come
the Soviet Union, Angola, Zaire, and Sierra Leone. Other important
areas are the Central African Republic, Tanzania, Ghana, and
Venezuela, with Australia and Brazil further behind, and India
now one of the last.
Value Diamond is one of the
most valuable stones, together with ruby and emerald. The
market value of diamonds is determined by complex grading
systems that divide them into a wide range of different cateGories.
The basic characteristics considered are weight, color, and
purity. Less crucial factors are the proportions of the cut
and surface finish.
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