There are quite a few theories on how opal was formed and I will try and give a summary of the most popular theories in as simple language as I can. I am not a chemist, nor a geologist, so I may get some details wrong but I hope you get the gist of the process.
Opal is basically made from silica with a water content varying between 6% and 10%. For those chemists among you the formula is SiO2nH2O. Silica is a very common element and we know it as sand or quartz. The hardness of opal is between 5.5 and 6.5 on the Mohrs scale of hardness which makes it a relatively soft material to cut compared to saphires, topaz etc.
But what gives an opal those wonderful plays of colour that is lacking in other forms of silica?
In 1964, the Australian scientist Dr J Sanders, made scanned electron micrographs of various types of opal. This effectively allowed him to see the microscopic composition of the opal. He found that opal is made up of minute particles of silica in closely packed spheres. These spheres are about one ten billionth of a metre in size and in gem opal are very uniform in size and are packed together in a very regular pattern. When light is shone on the surface it is broken up into its various wavelengths (difracted) and the different spectral colours are seen. In common opal the spheres are not regularly packed and this difraction does not occur.
How then was the opal formed in Australia?
Opal is found mainly in inland Australia which, many millions of years ago, consisted of a vast inland freshwater sea. There is plenty of evidence of this with many shells, skeletons of fish etc found over large areas. Around 110 million years ago, give or take a few million years, layers of kaolinitic clay was deposited on this sea floor. Kaolinitic clay comes from the weathering of feldspars which are a composition of various rock forming minerals which make up 60 % of the earths crust. This clay layer became what is known as the opal layer but at this time did not contain any opal.Then came a long period of sand being deposited on the sea bed with this eventually becoming a thick layer of sandstone as the sand was compressed by the weight of the deposits above it. During this period there were also many upheavals in the earths surface forming faults and joints in the various layers.
One theory is that as the sea dried up and the sandstone weathered due to sun and wind, water seeped down leaching out silicon from the sandstone and collecting traces of aluminium, magnesium, iron and other trace elements which were present and was eventually trapped by the kaolinic clay layer which was quite impermeable. Here it found its way into cracks and crevices and by a chemical process even replaced the shells and bones of various creatures whose bodies were buried in the clay layer. The faults and joints would have aided the flow of silicon enriched water and this could explain why there have been found greater concentrations of gem opal adjacent to these features. In fact many miners look for these in their search for opal.
Conditions had to be just right for this silicon enriched liquid to turn into opal. Researchers have agreed that certain conditions are required to form opal,namely
1. There must be a change from alkaline to acid environment.
2. There must be the presence of aluminium oxide, ferric oxide or magnesium oxide.
3. There must be the presence of sodium chloride or sodium sulphate
Apparently the conditions were met and opal was formed, as is evident by the fact that miners have located many areas of opal in these inland areas.
After the formation of the opal, weathering continued in the very harsh inland regions. The erosion and weathering was not uniform and this is evidenced by the hills and valleys throughout the region. The opal layer was relatively level in the areas where opal has been found. In parts of the opal fields the sandstone and opal layer were eroded away but in other parts they remained and the opal layer would be exposed to the air along this boundary. Pieces of opal from the opal layer would break off and wash away. As a result prospectors would eventually find some of them and work their way to where they would find the source of the opal. If they excavated into the hill the depth of the excavation would increase the further up the hill as they went and this explains the reasons for the varying depths of opal mines in the various fields.
This is all a very simplistic explanation of how opal was formed but it does give a picture of how the complexity and forces of nature worked together to form one of the worlds most beautiful of gems.
A website that explains this in more detail is Opals Down Under.
But, is this how it really happened?
E F Murphy, one of the first opal miners at White Cliffs and buyer of opal for Tullie Wollaston, the man who had the most influence in selling Australian opal overseas, had a different view on how the opal was formed and this was based on his observations at White Cliffs and Lightning Ridge. In his book 'They Struck Opal" he gives a very plausible account of how he believed opal was formed and I will repeat some of this in this post and you can make up your own mind.
"Opal is composed of silica and water, and silica is of volcanic origin; yet there is nothing volcanic in the cretaceous crust where opal is found. There is, however, rock of volcanic nature below the old seabed, and the silica came from there. In the course of time , this silica and water in solution generated boiling heat, like the water from the boiled springs in Iceland. During the cataclysms of the earth which are always going on, cracks or vertical seams formed in it. The silica solution, boiling hot, rushed up through these cracks, until it met the cretaceous crust of desert sandstone when it spread underneath and, after cooling off, left a deposit or seam of opal." .......
"In some cases it was able to reach the surface or near it, thus forming a surface patch. In its upward course anything it met with - shells, bones, corals or wood - became opal, the replacement sometimes being complete."
He then goes on to describe a particular patch of opal he had personally observed which supports his theory.
"In Saul's patch, the course of opal was easily traced. The big surface patch was an overflow. Where it came to the surface, there was a depression about 6 feet wide by 10 feet long. and the depression was filled with opal. From 3.5 inches thick in the centre it ran out to nothing on each side. When cleaned down on to the opal, the surface was as level as water." ......""Later a vertical seam was found showing its entry, and lower down there was more opal - and so on , down to the bottom of the sandstone."
He has other observations from his life in the opal fields which support his, and others, view that the silica came from below in a heated form and was forced upwards.
Who do we believe?
It doesn't really matter but it does show there can be more than one explanation to explain most phenomena. We are just lucky that somehow silica did turn into opal and that man was lucky enough to discover it.
There is also a theory that bacteria were involved in the formation of the opal but I will not go into this as I have also read reports that opal has been formed in the laboratory without the aid of bacteria.