Hollow or partly hollow generally subspherical bodies,
with inward pointing drusy crystals that grew into an
open space. Most geodes form in limestones or volcanic rocks,
and fewer form in shales and other rocks, and have an outer
shell of chalcedony or fine-grained silica. The crystals inside
geodes are often perfectly formed and may exhibit brilliant
colors depending on the chemistry of fluids that precipitated
the crystals and the impurities that became incorporated in
the crystal structure. Typical crystals in geodes include quartz,
calcite, barite, and various sulfide minerals.
See also DIAGENESIS.
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