Where do geodes come from? A simple explanation is that they started as bubbles in volcanic lava, like carbonation in soda. The molten rock solidifying around those bubbles formed hollow cavities. Because hardened lava is porous, water was able to permeate into the cavities. Over millennia, minerals in that water crystalized on the inner walls. The roundish black rocks look plain, but they are hollow and crusted inside with colored mineral crystals that are not plain at all.
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