A geode is a geological secondary formation within sedimentary and volcanic rocks. Geodes are hollow, vaguely spherical rocks, in which masses of mineral matter are secluded. The crystals are formed by the filling of vesicles in volcanic and subvolcanic rocks by minerals deposited from hydrothermal fluids; or by the dissolution of syn-genetic concretions and partial filling by the same or other minerals precipitated from water, groundwater, or hydrothermal fluids.
Quartz-filled geode, shown from inside (top) and outside (bottom)
Reddish chalcedony geodes
Very large amethyst and regular quartz geodes
Amethyst is a violet variety of quartz. The name comes from the Koine Greek αμέθυστος amethystos from α- a-, "not" and μεθύσκω methysko / μεθώ metho, "intoxicate", a reference to the belief that the stone protected its owner from drunkenness. Ancient Greeks wore amethyst and carved drinking vessels from it in the belief that it would prevent intoxication.
Amethyst cluster from Artigas, Uruguay
Natural purple/violet amethyst
Heat-treated amethyst
Natural pink amethyst