Facts And Myths
Some facts and myths about that beautiful opal you have locked away in your jewellery case.
What is Opal?
Opal is an amorphous non-crystalline gemstone comprised of hydrated silicon dioxide deposited within cracks or cavities. Opals do not have a definite chemical composition and are therefore mineraloids rather than minerals. Scientifically known as SiO2·nH2O, opals appearance ranges across the spectrum of light in a “play of colour” caused by the diffraction of light displaced by the tiny silica spheres that make up the microstructure of opal. This causes the colours reflected back to “dance” as the stones position is changed.
There are two common forms opal can take, precious opal and common opal (potch). Common opal occurs throughout the world in abundance and does not exhibit a “play of colour” within its faces. It is named potch when it is found with precious opal. Precious opal refers to opal that contains a “play of colour”. This term was created specifically to describe the lustre and spectral hues that constantly shift within an opal.
Opal is one of the world’s most desirable and expressive gemstones, found mainly in Australia it is classified as one the six types of precious gemstones. The hardness of opals is only 5.5 to 6.5 on the Moh’s scale. Because of this opal is best suited to jewellery that protects the stone from abrasions, such as brooches and earrings.
Why does Opal have so many colours?
Firstly, it is a closely placed array of millions of spherical particles of amorphous silica, stacked in a three-dimensional grating. Because of the special grating of the spheres, it is the only gemstone known to man that has the unique natural ability to diffract, that is to split white light or ordinary sunlight into all the beautiful colours of the rainbow.
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 void is 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.
Where does it come from?
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 person 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 opals. In all my research to date, I have not been able to get anything to penetrate our good opals, and am yet to meet the person who has.
How did it Form?
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.
Removing the water from an opal.
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.
Does soaking Opals in water help them shine brighter?
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.
The solving of these questions has shown the theory to be false, and that the opal was formed under very special conditions by a complex ion exchange process that can now be demonstrated in the laboratory. In geological terms, the opal grew quite quickly, picking up traces of the following elements at Lightning Ridge in the process.
Approximate Make-up of an Opal
6.0% H20 (Water)
2.5% Al2O2 (Aluminium Oxide)
0.9% CaO Calcium Oxide
0.4% Na20 (Sodium Oxide)
0.3% FE2O3 (Hematite – Iron)
0.1% TiO2 (Titanium Dioxide)
0.1% MgO (Magnesium Oxide)
0.02% ZrO2 (Zirconia)
0.02% MnO (Manganese Oxide)
0.006% CuO (Copper(II) oxide – Cupric oxide)
0.002% NiO (Nickel Oxide)
0.0025% CoO (Cobalt Oxide)