of Precious Opal: Anatomy of a Research
The Andamooka opal field,
some 600 km north of Adelaide, South Australia,
was discovered early in the 1930's. My uncle,
Ralph W. Segnit, was a geologist with the
South Australian Department of Mines at
that time, and was sent north to investigate
It turned out to be a wonderful opal field,
and, on his return, he presented me, as
a child of nine or ten years of age, with
samples of precious opal.
Thirty years later, I was working in the
then Division of Building Research of the
Commonwealth Scientific and Industrial Research
Organization, Australia, and I had occasion
to resurrect these old opal samples.
I arranged the testing of a number of mineral
samples, including one of my old opal specimens,
in the Differential Thermal Analysis equipment.
DTA, amongst other things, detects changes
in the sample, such as loss of water, or
change in the crystal structure, by rapid
changes in the temperature of the sample
and even heating. As opal contains several
percent of water, I naturally expected the
instrument to react to this loss on the
graph produced. It did not.
This seemed to be a false result, so the
experiment was repeated, but with the same
result. So came the change of direction
in what had started out as a survey rather
than a research project. A few other opal
samples which were available were tested;
some gave the expected result, some did
On a visit to the Department
of Geology at The University of Adelaide,
I contacted a colleague, Dr. J.B. Jones,
to beg a few more samples of opal. Dr. Jones
had scores of opal samples on the benches
in his laboratory. He had been studying
opal samples by X-ray
diffraction (XRD), and had been finding
unexplained differences between opal samples.
It was natural now that we pool resources
and try to iron out our problems together.
After examining a large number of samples
in various ways, we realised without doubt
that there were two or three distinct types
of opal. Nevertheless, there were still
some unexplained characteristics, especially
regarding precious opal. By a process of
elimination, we realised that the difference
must be due to something special about the
fine microstructure, but what could it be?
We found no solution by examination in the
optical microscope, which showed few clear
In the meantime we had obtained the help
of Dr. J.V. Sanders, one of Australia's
foremost electron microscopists, to examine
the fine microstructure of many of these
opals. We obtained many surprising results.
However, we still had the problem of unexplained
characteristics of precious opal. Armed
with one of those trusty specimens given
to me when a child, I once again visited
The next day came an excited call saying
that he had found in my sample a completely
new and unexpected
Better examples of precious opal were then
exhumed from collections, as well as samples
of potch (opal, from opal fields, showing
no colour play). Sanders obtained beautiful
electron micrographs showing tiny spheres
of silica stacked regularly together like
ball-bearings in a box.
It was immediately clear
that it was this regular structure which
was the basic cause of the colours in precious
opal. The structure formed a three dimensional
diffraction grating which 'reflected' (actually
'diffracted') only one wavelength (colour)
of light from the white light falling on
the opal. The diffracted colour would vary
according to the size of the spheres and
the angle of incidence of the light. These
results were first announced in the Annual
Report of the CSIRO Division of Building
Research for 1963 (R1633).
Once again, one of those
coincidences which occur from time to time
in scientific research happened. Professor
E. Baier, of the University of Mainz in
Germany, had studied the optical properties
of precious opal using a polarising microscope;
he published this work in 1932 (R1465).
From this work he deduced that the colour
of precious opal was due to diffraction
of light, but could not establish the true
underlying structure which gave rise to
However, in the early part of the 1960's,
he collaborated with Dr J. Pense, also from
Mainz, who produced electron micrographs
showing the periodicity of the microspheres
of silica in gem opal (R1561).
So the story of the colour of opal seemed
to have come to an end, with numerous scientists
having made positive contributions to the
problem over a period of more than 30 years.
But not so. About this time I happened upon
an old edition of the Encyclopedia Britannica,
about 1870 vintage, in a church meeting
room. On looking up the article on opal,
I found that the colour of precious opal
was caused by the diffraction of light from
a regular periodic structure in the stone.
This had been observed and essentially described
by Sir David Brewster, the Scottish scientist
famous for his work on wave motion and the
properties of light, in 1845! (Brewster's
paper is reproduced here)
It seemed impossible that Brewster could
have seen this periodicity in an optical
microscope, but next day, knowing what to
look for, I could confirm the observation.
An expert from the firm of Carl Zeiss later
told me that in the 19th century, special
objectives having only a small and curved
field of view but with very high resolution
at the centre, were made especially for
such people as Brewster.
We must admire the pertinacity and thoroughness
of the 19th century scientists. They had,
certainly in our terms, no sophisticated
equipment, but worked meticulously, and
gave much careful and intuitive thought
to the results they obtained.