whats the geology of Geods?

[distribuidorUSA]

I have a question for the rock&gems experts.we used to pick up Geods that were laying on the ground surface decades ago . Over the years they were harder to get.today all you find are scattered pieces of quartz. They look like old eroded pieces of veins.but there is a face cut on a hill to make way for an irrigation canal.and on this face I noticed what looked like conglomerate. It contained pebbles that ranged in size from less than an inch to give inches . But these rocks were actually small gross.but the material was light and soft.I could easily break them with one blow.inside there was a yellowish powder.no visible crystals. Is there a chance that deeper I might find the quartz Geods?. In those days we recovered beautiful Amethyst Geods.

 

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[Eu_citzen]

As for the geology:
Usually geodes form as an effect of trapped gas or something similar forming "holes" in molten lava/magma.
As the lava/magma crystallizes, hydrothermal solutions (hot water) can collect in these "holes", forming geodes.

 

[Jhaddad8]

That is one theory of geode formation, although it is under studied and still poorly understood.

 

[Eu_citzen]

Jhaddad, are there others?
So far this is the one I've heard of, especially considering the basalts of south America which turns out heaps of geodes.