the birth of opals: not too deep in the bedrock that makes the Earth there are all sorts of cracks. the Earth is always heaving and sighing and cracks form.
there is water also either soaking down from above or welling up from below. patchy heat sources can also send water seeping in from above back up.
these waters will then dissolve the minerals in the rocks. the most common minerals in rocks are silicates: quartz, feldspars, mica, clay...
so these waters become enriched in dissolved silica. silica is interesting stuff. silicon dioxide can arrange itself into all kinds of interesting structures: strands, rings, flat hexagonal networks... these molecules come with positively charged and negatively charged ends, that make them stick together tentatively like magnets into even more structures.
the way the silica structures attach to each other is modulated by the pH of the waters.
i don't really understand this... i must review my book:
"Silica and Me", by Guy Alexander. an excellent first person account of the adventures of a rookie research chemist in industry as he begins to solve some problems in silica chemistry. along the way he explains in a clear fashion a lot of interesting chemistry
(it appears to be out of print but a few copies appear to be available from amazon at the moment)
under certain conditions of temperature, pH and other dissolved ions like carbonates and sodium etc... the silica will precipitate out as crystals or spheres i don't know what selects for either... perhaps crystals precipitate out only in purer silicon dioxide, while being mixed with water produces a kind of silica tetrahedron strand - water chaos...
now the microspheres 150nm to 1000nm wide will be in suspension and will be negatively charged and will repel each other. by modulating the ph and ion content and concentration of the spheres some more the degree to which the spheres can contact can be modulated. under some conditions they will want to contact the least and will thus form long linear strands. at point of contact though, chemical bonding can occur. as concentration goes up these strands can precipitate and line up, again contacting the least amount to make perfect cubic packing of the spheres
if there are spheres of different sizes... they will tend to come into closer contact with each other... (draw a picture) (actually, if there are small spheres that can fit between the spaces in the larger sphere packing that would be the least favorable situation, but a bunch of spheres of SLIGHTLY different size would produce less perfect packing and so less contact? hmm this is puzzling, requires more thought) so perhaps electrostatic repulsion alone can cause spheres of uniform size to precipitate together...
so you can have a multistrand strands of uniform sized spheres of one size next to another multistrand strand of uniform spheres of another size...
it is the regular arrangement of the these uniform spheres that forms a diffraction grating that causes the color play in the opals. so you can have streaks of different color/opalescence this way
other processes that can effect opal formation are other trace minerals to form nuclei around which the microspheres first precipitate out.
another process is sorting of sphere sizes by filtration through clays by pressurized silica solutions. the pressurization could come from volcanic heat or from other clays. some clays will absorb water and this causes them to swell, so they will exert pressure on adjacent layers of silica solutions...
another factor that can effect the precipitation of microspheres is if cooler waters or waters of different pH seep down and contact the warm waters rich in dissolved silica.
another factor may be bacteria living in the water filled cracks. they may also be able to modulate chemical conditions around themselves to effect the formation of these microspheres.
where i got some details from:
THE ORIGIN OF PRECIOUS OPAL: A new model