Geode

A geode is a rounded, hollow rock lined on the inside with crystals. From the outside geodes look like ordinary lumpy stones, but split open they reveal a cavity glittering with inward-pointing crystals — most commonly quartz and its purple variety amethyst, but also calcite, celestine and banded agate.

How geodes form

Geodes begin as cavities in rock — gas bubbles in volcanic lava, or voids left by buried roots, shells or nodules in sedimentary rock. Over long periods, mineral-rich groundwater seeps in and deposits successive layers on the walls: often an outer rind of chalcedony or agate first, then a lining of crystals growing inward. If the cavity fills completely it becomes a solid nodule rather than a hollow geode.

Types and famous sources

Amethyst geodes and large amethyst 'cathedrals' come mainly from Brazil and Uruguay; agate-lined thunder eggs from volcanic regions such as Oregon; and celestine geodes are famous from Ohio. Sizes range from a few centimetres to room-sized.

For collectors

Unbroken whole geodes are sold to crack open at home, while sawn-and-polished halves showing sparkling crystal interiors are popular display pieces. Crystal quality, colour and an attractive cavity drive value.