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Posted by: Larry Maltby
“Agates form in volcanic rocks or volcanic ash related to volcanism, only.”
This is an extremely absolute statement. It leaves no “wiggle room”. It implies that all of the complex geological processes in the world have been investigated and this is the conclusion. The formation of agate remains as a highly controversial subject with some good points on all sides of the debate but as yet it is far from a conclusion. It seems to me that agates sometimes form in sedimentary deposits as a chemical process that is separate and apart from volcanism. It may well be that the silica in a sedimentary agate was derived from the decomposition of an overlaying layer of volcanic ash but the ash fall itself is a sedimentary process and the decomposition of the ash that frees the silica is not a volcanic process. There are other sources in sedimentary deposits that may provide silica to support the formation of agate such as plankton, diatoms etc.
Really, there is no such thing as a simple explanation of agate formation!