& zircon grains
Properties: Opal & Light
We are not
concerned here with the special characteristics
of precious opal. The development of colour
in the gem variety is dealt with in the
section The Colour of
Opal. One of the main optical properties
of the whole family of opal, and of transparent
minerals in general, is the refractive
Mineralogists have traditionally determined
this property of non-opaque minerals and
other materials by immersion of a powder
in liquids of known refractive index (the
method). Until the advent of X-ray diffraction
(XRD), the determination of optical properties,
especially refractive index, was the most
useful method of identifying minerals. Even
today, it can help to distinguish between
minerals of similar structure which give
similar XRD patterns.
Using the immersion method, the refractive
index can be determined for most materials
to three decimal places. For isotropic materials
(those with only one refractive index) it
is possible, with care, to measure the index
to a fourth decimal place.
When enough material was available, prisms
could be cut which enabled the refractive
indices to be determined with great accuracy
using optical instruments. The optical mineralogists
of the 19th century frequently quoted such
figures, and there is every reason to believe
that their determinations were substantially
In relation to opal, Kokta (R1582)
collected published data from earlier literature
in order to see if there was a correlation
between the composition, especially the
water content, of opals, and their refractive
indices. There were in fact, few investigations
relating water content to optical properties.
Kokta himself measured the refractive indices
on the 18 opals for which he determined
water content and density, and showed that
there was a broad relationship - refractive
index increasing with decreasing water content.
are plotted as a graph. The scatter
of the points is probably due to the differing
physical characteristics of the samples
mentioned earlier, and perhaps the presence
of 'impurity' elements.
It can be seen that the
refractive indices of most opals lie within
a fairly narrow range between 1.44 and 1.46,
although occasional samples with high water
contents will have lower indices.
Another property of non-cubic non-opaque
crystalline materials is that called double
refraction, or birefringence. Such materials
have the property of splitting a transmitted
light beam into two polarised rays.
This phenomenon is observed when a thin
section (a thin slice, usually 30 Ám, or
0.03 mm thick footnote,
mounted on a glass slide) of the material
is viewed in a petrological microscope between
crossed polarisers. The polarised rays of
the transmitted light cause interference
which are seen as white
or coloured effects in the crystals
of the thin section. Cubic and amorphous
materials do not show this phenomenon; when
viewed in the same manner they remain dark,
although precious opal is an exception to
Some common opals show this phenomenon.
As indicated in the topic, Classification,
most common opal is crystalline (opal-CT).
In most cases the crystallites are too small
for birefringence phenomena to be seen.
However, sometimes the crystallites have
grown sufficiently large for birefringence
effects to be seen. These usually show
that the opal is composed of tiny, fibrous
This structure was first described by Mallard
in the late nineteenth century; he called
such opal by the name of 'lussatite'. This
term is reappearing in the literature today,
being recognised as a useful terminology.
A property of precious opal which is seldom
recognised is its propensity to show a pseudo-birefringence
in thin sections, especially if the slice
is made slightly thicker than usual. When
viewed in the microscope between crossed
polarisers, bright colours similar to those
found in some crystalline minerals are seen.
This is caused because some of the light
passing through the opal is diffracted,
but at the same time, it is polarised. When
the plane of polarisation of the diffracted
beams is not parallel with the axes of the
polarisers of the microscope, the coloured
rays can pass through and be seen by
the observer. Any non-diffracted light is
extinguished by the crossed polarisers,
thus increasing the apparent intensity of
the coloured rays.