Amethyst and calcite geode from the Los Catalanes district, ready for sale. Credit: Fiorella Arduin Rode The amethyst geodes, of rare beauty, form in volcanic rock cavities. How did these purple ...

They look a bit like geodes, but they’re not exactly geodes. There are three types of rock: igneous, metamorphic, and ...

Geodes are hollow rock formations often with quartz crystals, such as amethyst, inside. Amethyst geodes in Uruguay have been found in cooled lava flows, which date from the original breakup of the ...