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Sapphire 2
Appreciable quantities of light- and bright-blue
sapphire are found in alluvial deposits on the island of Sri
Lanka. These are always attractively (if sometimes patchily)
colored, the richest versions being very similar to the Burmese
sapphires and equally valuable. The sapphires of Sri Lanka
are also famous for the variety of inclusions they display:
long, thin rutile needles, like very fine silk; soft, liquid
inclusions arranged in the form of veils, lace, and feathers;
striking inclusions with a moving bubble, like a spirit level;
zircon crystals with small stress cracks radiating from them,
and various other types of transparent crystals.
Sapphires are also mined in Thailand and neighboring Cambodia.
These are generally pleasing to the eye, though often rather
deeply colored. But most sapphires come from Australia, which
has numerous deposits of deeply colored stones, sometimes
too dark, in most cases with blue-green pleochroism. These
are the least valuable, but most widely available on the market.
Less important sources are the United States (Montana), Tanzania,
and Malawi.
Value The finest stones, weighing
at least several carats, are almost as valuable as diamonds
and rubies and are hence very highly priced. This is particularly
true of most sapphires from Kashmir, many from Burma, and
some from Sri Lanka, Cambodia, and Thailand. But when the
color is too dark, blackish or greenish blue or a bit too
pale, the value falls sharply, to that normal for secondary
gems. Inclusions obvious to the naked eye also lower the price.
Small stones (of a fraction of a carat) are modestly priced
and readily available. Large ones (from more than ten to several
tens of carats), although not common, are much less rare than
rubies of this size.
Simulants and synthetics Sapphire
has been imitated by dark to cobalt blue glass, but particularly
by doublets with a top part consisting of red almandine garnet,
which is very hard and lustrous, with natural inclusions,
and a bottom part of dark-to-cobalt blue glass, welded together,
not glued. It has also been imitated in the past by synthetic
blue spinel, which is brightly colored but emits strange red
gleams in bright light. Synthetic sapphire has likewise been
produced for many years now, mainly by the Verneuil flame
fusion method. Recently, doublets have been produced consisting
of a top portion of light green or yellow-green natural corundum
with visible inclusions and a lower portion of synthetic sapphire,
held together by transparent cement. The visible inclusions
and typical corundum of the top part, along with the color,
make these doublets very convincing at first sight.
Since the end of the 1970s, greater knowledge
of the nature and causes of color in gemstones has enabled
the modification of this feature by various procedures. The
most recent and important techniques, in fact, relate to the
blue coloration of sapphire. One method is to subject very
pale blue, almost colorless stones with numerous silklike
rutile inclusions to prolonged heating at temperatures ir
the region of 1500-1600°C. in a reducing environment This
"reactivates" the titanium in the rutile, which
reacts with the traces of iron in the sapphire. In this way,
the silk is absorbed, while the trivalent titanium and iron
thus formed which are responsible for the blue coloration
of sapphire, greatly intensify the color of the stone. This
treatment is now very widespread and more or less reproduces
the sequence of events that occurred when many sapphire crystals
were formed.
As a result, it is not always possible to distinguish
a completely natural sapphire from one whose color has been
intensified in this way, and they are treated as one on the
market. According to another procedure, however, colorless,
pale yellow or pale green stones are covered in a paste consisting
of iron and-mainly-titanium compounds. The specimens are then
heated to a temperature of about 1700°C. for perhaps several
days. The iron and titanium oxides slowly infiltrate the stones
to a depth of about one millimeter, producing a deep blue
coloration. The stone then has to be repolished (the surface
having been damaged by heating to near melting point). Hence
part of the colored layer is removed, leaving a very small
thickness. This procedure is surprisingly common and involves
the introduction of additives as colorants. It is universally
regarded as fraudulent if the treated stones are then offered
for sale as natural stones, as is often the case. Inclusions
in the shape of a butterfly's wing in a blue sapphire
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