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Posted by: Donald Kasper
We can estimate agate formation by understanding they only form in lava flows and pyroclastic rocks. So we know the cool-down time of those flows based on their type and mass (thickness). So we can make estimates of formation time. So, you have an ash flow 30 meters thick. You have a certain cool down rate. This has been studied for Mono Lake flows. Then you take a theory of formation at 374 to 575 C which is both a rhyolitc and quartz state polymorph transistions. From that, you have 8 days. I don't call that weathering.
Then you go over to marine rocks. You get opal-A from forams. You slows bury this in muck and heat it to form opal-CT, linked to agate formation. You have up to 70 million years for the very coldest conditions of formation. That is it. So the relation is time-temperature dependent. But, the agates in those sedimentary rocks take a microscope to see. You have to understand that a 100 micron agate is not the same as a volcanic agates. For example, no dilation tubes of exit/escape exist on this scale. In fact, don't exist for any agates under about 3/8th of an inch. So the silica banding is different in these sedimentary "agates" which are more like veinlets of banded silica on a mm scale.
Since these form in subcritical conditions, these "agates" never have waterlines. They never have the calcite and celadonite and other mineral inclusions. They are another class. The scale of your classification matters, and overgeneralizing causes confusion because as you overgeneralize, you combine different geologic systems and then get a confusion of superficially conflicting data.
Overall, in volcanic systems, agates are forming in under 30,000 years. This is why you cannot dig in your back yard to study the soil weathering profile and find bigger agates with depth. There are no agates in weathering profiles.