Quartz Geodes can form in any cavity, but the term geodes is usually reserved for more or less rounded formations in igneous and sedimentary rocks. Quartz geodes can form in gas bubbles in igneous rocks, such as vesicles in basaltic lavas, or as in the American Midwest, rounded cavities in sedimentary formations. After rock around the cavity hardens, dissolved silicates and/or carbonates are deposited on the inside surface. Over time, this slow feed of mineral constituents from groundwater or hydrothermal solutions allows crystals to form inside the hollow chamber. Bedrock containing quartz geodes eventually weathers and decomposes, leaving the quartz geodes present at the surface as quartz is a particularly resistant material to weathering.