Geodes are small rock cavities lined with crystals or minerals. They form through sedimentation, a process in which particles (solute) are suspended in fluid (solvent) and then accumulate. (Here, the ...
Amethyst geodes—some stretching 5 m long—dot the ancient lava flows of Uruguay’s Los Catalanes mining area. The geodes contain calcite, colorless quartz, and agate, another quartz mineral. Large ...