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Blue Topaz
This is the variety of topaz most readily
available on the market today.
Appearance It has a definite,
uniform sky-blue color usually without any overtones. Often
pale, it can be bright or very rarely an intense blue. It
sometimes has a slight gray or even greenish tinge, giving
it a lifeless appearance. Gemstones of several carats or even
several tens of carats in weight are relatively common. Furthermore,
they are usually wholly or almost free of inclusions. This
is often the case where large amounts of material of no great
value are available; the less clear pieces are discarded.
The most common cut is the oval, with the crown and pavilion
consisting of very many lozenge-shaped facets, but all w mixed
cuts, plus the step cut, are used. As with all light colored
gemstones, the value of blue topaz increases with tensity
of color, provided this is attractive and not somber. Like
other types of topaz, it cleaves readily and this can at fect
its durability.
Distinctive features There
is, at first sight, some resamblance to aquamarine; but close
observation will distinguish the two, as aquamarine always
display very attractive pleochroism from blue to greenish
blue or even bluish green. Topaz is usually a more definite
blue, if any thing with a grayish tone, which certainly distinguishes
from aquamarine. Measurement of the density alone is - enough
to distinguish topaz from synthetic blue spinel which can
be very similar in color. Before measuring other physical
properties, such as the refractive indices, the stone can
be examined under a lens for signs of birefringence. Although
faint, a doubling of the facet edges will b, visible in topaz.
If this is present, one can immediately rule out the possibility
of its being synthetic spinet or glass both of which are singly
refractive.
Occurrence Blue topaz is
found in various parts of Brazil, Mexico, and the United States.
It is mined in Burma (in the Mogok region famous for rubies)
and the Soviet Union (chiefly in the Urals and the Kamchatka
peninsula). It is also found in Namibia and Nigeria.
Value Quite low, several
times less than that of aquamarine. The ready availability
of blue topaz on the market even in pieces of considerable
size, is probably responsible for this.
Simulants and synthetics
Because blue topaz is a relatively minor gemstone compared
with aquamarine, it is the latter that is imitated by glass
or synthetic blue spinel. AIthough blue topaz has not been
manufactured synthetically on a commercial scale, a completely
natural-looking blue coloration has been produced during recent
years in colorless topaz by means of irradiation. This practice,
regarded as legitimate in the trade, unless performed in such
a way as to cause appreciable residual radiation (fortunately
very uncommon), is becoming increasingly widespread and is
one of the reasons for the present abundance of blue topaz.
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