HOW MEXICAN GEODES FORM
STEP 1: The spheres that become beautiful geodes begin as bubbles formed in lava flow.
STEP 2: The bubble is cast in a hard shell of silica and water containing dissolved minerals is trapped inside.
STEP 3: The precipitate can contain a wide variety of minerals, usually quartz or calcite, but also aragonite, magnetite, barite and chalcopyrite.
STEP 4: The precipitate forms a lining of inward projecting crystals and since different minerals harden at varying temperatures, the material forms layers, which result in beautiful outer banding and inner crystalline structure.
Sometimes a cavity may be completely filled by layers, which is referred to as “nodules”
STEP 5: Over thousands of years, weathering lifts the, now formed geode, from the surrounding rock.