Calcite is the mineral commonly forming stalactites and stalagmites in limestone caves.
Rainwater absorbs carbon dioxide from soil and becomes mildly acidic. As this water moves through limestone, it dissolves some calcium carbonate. When the water enters a cave, carbon dioxide escapes and the solution can no longer hold as much dissolved material, so calcite precipitates.
A stalactite grows downward from a cave ceiling, while a stalagmite grows upward from the floor. A useful memory aid is that stalactites hold on tightly to the ceiling, whereas stalagmites might reach the ceiling. If the two formations eventually join, they create a column.
Calcite and aragonite have the same chemical formula, CaCO₃, but different crystal structures. Calcite is the more common mineral in ordinary cave deposits. Cave formations may also contain impurities that create white, amber, brown, or reddish colors, but the principal deposited mineral is generally calcite.