How do crystals form in rocks?
(Lansing State Journal, July 20, 1994)
The fantastic crystal structures in geodes, or hydrolites, form as water evaporates from the internal spaces of the stones. However, most crystals are found in rocks that have a volcanic origin. As the super hot molten rock - called magma - cools, different substances will crystallize out. Sometimes very large crystals, weighing up to several tons, can be made as magma slowly cools below the surface of the Earth.
Crystals also can be formed when land masses move. The crust of
the earth can buckle and fold, raising mountains and creating new faults.
Such stresses create great pressures and temperatures that can change the
crystal structures in rocks. These rocks are called metamorphic rocks.
Crystals in metamorphic rocks are generally pretty small.