Geode

Hardness of 7 on the Mohs scale for the quartz/chalcedony interior; outer rind is typically harder limestone or basalt (hardness 4-6). Colors vary widely including grey, brown, white, clear, purple, or multicolored banding. Luster is vitreous to dull. Cryptocrystalline structure inside with macrocrystalline quartz crystals lining the central cavity.

Formed in hollow spaces in sedimentary or igneous rocks (such as vesicles in lava or cavities in limestone) where silica-rich groundwater percolated through over long periods, slowly depositing microcrystalline quartz along the walls.

Popular in lapidary arts, specimen collecting, making decorative bookends, and jewelry making.

Geodes start as empty nodules or gas bubbles in rock. Over millions of years, mineral-rich water deposits microscopic crystals layer by layer, often leaving a hollow center lined with sparkling quartz crystals.

Commonly found in limestone formations, deserts, and volcanic regions worldwide. Collectors often look for rounded, unusually lightweight nodular rocks that feel heavy for their size or sound hollow when tapped.