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Impurities play no part in creating the
colours of the opal as is the case with many other gemstones.
Colour is created when light is split by voids that have been
formed between the spheres. The size of the voids are critical
to the angle at which the light is split, hence the colour
produced. To produce colour visible to our eyes, the spheres
that form the voids must be no smaller than 1500 angstroms
for the violet, indigo and blue colours, nor larger than 3500
angstroms for the orange and red.
When viewing a lovely opal the most critical
factor is the relationship of the light source to your eyes.
To see it at its sparkling best the light must be coming over
your shoulder. e.g. If you are in a room using natural light
always stand with your back to the window and if outdoors
stand with your back to the sun. Reverse these positions and
you will be amazed at the change of quality and colour. The
same principle applies to most other forms of gems, hence
the critical angle of light in the jeweller's shop.
Good stable Australian opal has proved
to be a sound investment over the past 25 years. It is one
of the few gemstones that has continued to appreciate in value
at a steady rate. This cannot be said for many other types
of gems. For example, I can recall the time when I would have
had to pay around $60,000 for a first water one carat diamond
and two years later could have bought the same grade stone
for $14,000. This has not been the case with gem grade Australian
opals, especially black stones.
Australia has some 96% of the world's supply
of commercial grade opal and suffers little of the problems
that seem to apply to many forms of opal from other countries.
This is due to the fact that it has been laid down under very
special sedimentary conditions.
Many of the problems of other Countries'
opals have become myths with Australian opals. Possibly, the
most common being that you must not let oil, or any of the
usual household products that a woman may use around the home
get on your opals, as they will penetrate it causing irreparable
damage. Though this may be the case with some forms of opal
from other countries, it does not apply to good Australian
opal. In all my research to date, I have not been able to
get anything to penetrate our good opal, and am yet to meet
' the person who has.
Another popular myth is that you should
soak your opal in water every so often. Again if it is not
a form of hydroplane and is a good solid Australian opal you
are just wasting your time.
Go ahead and soak it if you like, but you
won't enhance it one little bit.
The average Australian opals from Lightning
Ridge, Coober Pedy and Andamooka carry around 6% water, most
of which is locked up in tiny voids between the spheres. These
voids are so small that it requires an electronic microscope
to see them. Under these conditions the water molecule is
held so tight that it is near impossible for it to escape
without some help. It may be well to point out that every
20,000 layers of voids is equivalent only to the thickness
of a fine dot made by a ball point pen.
It requires about 60 degrees Celsius to
start to remove water from the first few exposed layers of
voids and to continue moving it to its zenith rate of evaporation
requires a steady temperature of 250 degrees Celsius. Should
you wish to remove all the water from your beautiful opal,
then you will need to raise the temperature for some time
to 600 degrees Celsius plus.
Should you ever try this experiment and
use the correct procedure for removing the water, you will
find that your opal has lost all of its colour. This, is due
to the change in internal refractive index. Try, as you may,
you will find it near impossible to get the water back into
your opal, even over long periods of time with the help of
vacuum or pressure.
Another myth is the depositing and genesis
of Australian opals. The accepted scientific theory, based
on the Geologists uniformitarian belief, is a slow and gradual
process over millions of years where by silica gel, a warm
water solution, saturated at 120 parts per million was deposited
layer after layer. This theory has always carried with it
unanswered questions that have been pushed into the background
so as to give it some credence.
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