| There
is some doubt that a true synthetic opal
of gem quality has been made, although stones
have been made which are very similar in
composition and properties. Essential to
this must be the internal structure of close
packed spheres which cause the development
of colour by diffraction of the incident
light beam.
Some of the earliest attempts to make synthetic
opal were carried out in the Melbourne CSIRO
laboratories under the direction of A.J.
Gaskin. Once it was known that precious
opal was built up of minute spheres of silica
(R0362),
it was a short step to the realisation that
this structure could be duplicated by polymerisation
of silica sols of appropriate composition
to form silica spheres of appropriate size,
with subsequent settling into a regularly
packed form. This was achieved without much
difficulty, as the behaviour of these sols
was well known from the work of Iler (R0338).
Opal-like silica showing the same diffraction
colours as natural opal was produced, but
difficulty was encountered when trying to
dry the material to an acceptable hard product.
The synthetic material invariably cracked
and turned milky.
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Most of these difficulties
appeared to have been solved by Pierre Gilson
of Switzerland, who produced opals remarkably
like natural materials, ranging from transparent,
through milky
to black opals. The stones were very
attractive, stable, and could be made as
cabochons or triplets. When mounted, many
of the stones which had a similar pattern
to the natural material could pass as natural
to the non-professional. However, careful
examination of the stones showed small differences.
For example, the boundaries between colour
patches tend to be straight in the natural
stone, while they are crenulated in the
Gilson material. This is in part caused
by the curious mosaic pattern seen within
the colour patches, and termed the 'lizard
skin' effect.
Detailed work on the Gilson stones by Schmetzer
et al (R1009,
R1631)
has shown that many, if not most of them,
differ somewhat from the natural opal. Many
contain a small amount of zirconium oxide,
the function of which is not fully understood,
and some contain little or no water. These
differences would therefore put some, if
not most of the Gilson material into the
category of 'artificial' opal. Jobbins et
al (R1630)
have also studied the structures to be found
in Gilson opal using optical microscopy,
and have described in detail how they may
be identified. Nevertheless, the Gilson
product is the nearest, and certainly the
best, commercial artificial opal yet to
be made.
Some remarkable experiments have been carried
out by Mr Len Cram, of Lightning Ridge.
He has produced opaline material by long
term reaction of natural materials with
other substances in glass jars. This is
undoubtedly the closest simulation
of the growth of opal yet to be achieved.
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