A geode is usually a hollow, more or less spherical or ball-shaped shell
of mineral and crystal growth which has formed within surrounding rock.
Missouri geodes commonly vary in size from hickory nuts to small
watermelons, although neither direction of variation is limited. They
weather out abundantly at several localities in northeast Missouri from
the so-called Warsaw formation, a limy shale. Here they are dark brown,
rough and irregular on the outside, but where broken open show many
brilliant, glistening faces of intergrown quartz crystals. Less
frequently calcite, chalcedony, kaolinite, and rarely millerite (nickel
sulphide) may occur in Missouri geodes.
[Illustration: Typical small geode from northeastern Missouri.]
The minerals and crystals of a geode grow inward from the walls of a
cavity in the rocks. The mineral matter is carried there in solution by
ground water and crystallizes out very much more slowly but in the same
manner that sugar or salt crystals develop in a saturated solution of
those substances. If crystal growth continues until the geode is solid,
it may bear superficial resemblance to a concretion, but the latter
structure is one which has grown outward. The idea of “growth” in either
case is that of mineral crystallization and enlargement, but does not in
the least involve life like that of a plant or animal. Geodes have no
value or use other than for ornamental purposes.