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You can see the tiny but long fracture in this hollow Coyomito Agate nodule where the solutions may have entered to deposit the silica, forming the layers of agate (concept of "open system"). That shows how water can easily flow through any fractures (no matter how small) within any rocks.       A place at Arizona I know has a well pentrating a large grandiorite igneous body. The water went through that igneous rock very easily because of so many fractures (some are quartz-flled) within the igneous rock body. The agates also seem to be concentrated around the fissures where water may have flowed easily.