Quartz Geode

Hardness: 7 (Mohs scale); Color: White to colorless (clear) crystals, tan to brown outer shell; Luster: Vitreous/glassy; Crystal Structure: Trigonal/Hexagonal; Cleavage: None/Conchoidal fracture; Specific Gravity: 2.65

Formed when gas bubbles or chemical cavities in sedimentary rock (often limestone) or volcanic rock are filled with mineral-rich hydrothermal fluids. Over thousands of years, these fluids precipitate crystals along the interior walls. Most are millions of years old, often originating in the Cretaceous or Jurassic periods.

Used primarily for geological education, home decor, rock collecting, and metaphysical/spiritual practices. Some larger high-quality crystals may be extracted for industrial silicon production or lapidary work.

Geodes are often referred to as 'thunder eggs' when they are more solid. The world's largest geode is the 'Pulpí Geode' in Spain, which is large enough for several people to stand inside. Some quartz geodes contain secondary minerals like calcite or hematite inside the cavity.

Identify in the field by looking for rounded, lumpy nodules with a distinct cauliflower-like exterior texture that feels lighter than a solid rock of its size. Common in desert regions (USA, Mexico, Morocco). Tips: A light tap can determine hollow versus solid; hollow ones produce a dull 'thud' and are more valuable.