ThundereggsAs nearly as I can make out, these rocks are similar to geodes or nodules, usually filled

ThundereggsAs nearly as I can make out, these rocks are similar to geodes or nodules, usually filled with varied forms of microcrystalline silica rather than quartz crystals, and formed in silica rich lavas rather than basalts (such as the agate nodules that dot like the Colombia River basalts in the western USA or the ones I pick up by the kilo in northern Uruguay from the Parana flood basalt, the residue of the beginning of Africa’s separation from South America). Those with crystal filled holes in the middle qualify as geodes as well. Both kinds form in whichever lava as waters remobilise silica through the volcanic rocks, depositing it in ex bubbles and cavities in the frozen lava (hence they are often spherical). The colours depend upon the changing trace element content of the fluids, picked up from the rocks that they have flowed through. They range up to metre sized and form many patterns like the amazing bird shaped agate in this specimen that formed in rhyolite erupted in Mexico.LozImage credit: Captain Tenneal -- source link
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