Geodes (in case you're wondering, they're the rocks that look like plain rocks on the outside, but have shiny crystals on the inside) form in both sedimentary (rocks formed from particles deposited by water or air) and
igneous (rocks formed from lava) rock. While it's possible for geodes to form in certain metamorphic rocks (rocks made from other rocks due to heat/pressure/etc.), crystals formed within metamorphic rocks tend to be quite small.
Both are pottery conducive to it, as all they really need is porous rock with a cavity. Since sedimentary rock is more common, I imagine more geodes are sedimentary geodes.
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