Crystal Geodes begin as bubbles in volcanic rock or as animals burrow, tree roots or mud balls in sedimentary rock. Over time the shap hardens and water containing silica hardens.it forms on the inside walls hollowing the cavity. Over 1000s of years layers of the silica cools, forming crystals of different minerals. ‘each one is unique and is like a surprise when opened. They come already split for you to revile your geode.