Geodes are cavities in rocks that are filled with beautiful crystals. The best way to describe how they form is to think of them as gas bubbles that became trapped in the rock as it cooled. Later, mineral-bearing fluids can seep into the gaps and solidify to either fill or partially fill the cavities. Some of the most impressive geodes are the purple amethyst-lined examples from areas like Brazil or multicoloured agates from Scotland.
Answered by Peter Davidson, Mineralogy curator, National Museums Scotland.