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[VegasJames]

Just finished a write up on opal to clear up some of the confusion about  it.

General Facts About Opal

-There are around 300 types of opal separated in to two primary categories.  Precious and common.  Precious opal is any opal with a play or color as where common opal is any opal without a play of color. 

-Precious opal consists of smaller silica spheres in an ordered arrangement. The size of the spheres determines the colors in the flash.  Common opal consists of varying sizes of silica spheres in a disordered arrangement.

-Opal is generally considered amorphous, which means lacking crystalline structure.  Although opal can contain crystalline compounds known as cristobalite and tridymite.  If the opal contain only cristobalite then the opal is referred to as Opal-C.  If the opal contains both cristobalite and tridymite then the opal is referred to as Opal-CT.  Completely amorphous opals are referred to as Opal-AG (amorphous gel) such as precious opal and Opal-AN (amorphous network) such as hyalite opal.

-The general chemical composition of opal is amorphous silicon dioxide (silica) although opal can also contain varying amounts of aluminum oxide as well as trace amounts of elements and other compounds.  The aluminum oxide comes from the clay most opal forms from. The higher the aluminum oxide content the harder and more stable the opal is.

-Opal is also found on the tips of bee stingers and the sharp points of the stinging nettle plant.  Opal can also form from the silica rich bamboo sap and is referred to as “vegetable opal”.

-Contrary to popular geological teachings opal does not take millions of years to form.  It has been shown that opal can form in very short periods of time, less than one year.

-Opal starts as a silica gel.  The silica molecules naturally attract towards each other squeezing out water in the process. When sufficient water is squeezed out opal is produced.  If all the water in the opal is lost then the opal converts in to a chalcedony including jasper or agate, or can convert in to chert.

-Because chalcedonies can form from opal you can also find examples of opal mixed with chalcedonies as part of the opal has transitioned.

[Felicia]

Seriously interesting. Been stings and stinging nettles! Next time I'm stung by a nettle, at least it will have used a gemstone to sting a rockhound.  Lol

[Kaljaia]

So... more or less what coats the windows and toilet bowls in my folks' house on the silica-rich water system?

But yes that's a great writeup! Thanks for posting it. I said something about "Precious" and "Fire" opal on a facebook post where a gal had a common but clear opal chunk with a good sheen in it and boy did I earn myself a lecture from someone there. I have found numerous thunder eggs with common opal healed fractures, and have wondered what the timeline was for those filling in. It could be a lot more recent than I thought, based on this; and is a process that is probably still happening. You don't usually think of rock as dynamic (on our time scale) apart from weathering down.

[VegasJames]

So... more or less what coats the windows and toilet bowls in my folks' house on the silica-rich water system?

But yes that's a great writeup! Thanks for posting it. I said something about "Precious" and "Fire" opal on a facebook post where a gal had a common but clear opal chunk with a good sheen in it and boy did I earn myself a lecture from someone there. I have found numerous thunder eggs with common opal healed fractures, and have wondered what the timeline was for those filling in. It could be a lot more recent than I thought, based on this; and is a process that is probably still happening. You don't usually think of rock as dynamic (on our time scale) apart from weathering down.
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The build up in toilets and on windows is usually calcium salts.

Glass around showers can also etch due to the water dissociating forming hydroxides that can etch the glass.

There is a guy in Australia that has been making precious opal from the opal dirt around the opal mines there. His opal cannot be differentiated from the naturally occurring opal. He says it takes about 6 months to grow, then another 6 months to harden.  I have been playing with making opal for years, but have only been able to form common and jelly opals. Still working on precious opal but feel I am getting much closer.

There is a case he discuses about a famous case where a miner's cat dies and the miner wraps the cat in his hat and places the dead cat in the opal mine. The mine sits dormant for about 50 years until bought by another miner. When they go in the mine they find the bones of the cat, which have been opalized with pink opal.

He also mentions the various cases of wood fence posts dug in to opal dirt that in relatively short periods of time have opalized below ground.

[womanwithatorch]

Thank you for the write up. What about the term "opalized"? Like petrified but with silica?

[lithicbeads]

Opal is amorphous silica while calcedony is very ordered and resistant. Wood replacement can be either.