.

    Once the channel gets clogged, an agate nodule couldn't grow anymore. The agate bands tend to be porous while quartz crystals tend to be not, because the quartz crystals were growing tightly against each other while agate microcrystals were growing in radiating structure that resulted in the gaps (hence, porosity) between the fiber-like crystals. Thus, the agates are much easier to be dyed than the quartz crystals. Anything even water can be trapped within a cavity by quartz crystals as in case of "Enhydro" Agate containg an air bubble was trapped inside its water-filled geode.  It seem to me that the endryo agate slowly start to lose water over time as the water escape through between quartz crystals and pores within the agate which account for an "bubble" surrounded by air trapped within a geode and it was demonstrated by leaving it out in arid eniviroment of Arizona.