mineral
Quartz geode (likely amethyst-bearing)
Silica geode; quartz, SiO₂, with possible amethyst and minor iron oxides
AI-generated identification · may be incorrectA hollow volcanic-rock cavity lined with sparkling quartz crystals; gray-white chalcedony and drusy quartz surround darker purple areas. Quartz is Mohs 7, vitreous-lustrous, trigonal, with indistinct cleavage and specific gravity about 2.65.
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Physical properties
A hollow volcanic-rock cavity lined with sparkling quartz crystals; gray-white chalcedony and drusy quartz surround darker purple areas. Quartz is Mohs 7, vitreous-lustrous, trigonal, with indistinct cleavage and specific gravity about 2.65.
Formation & geological history
Forms when gas bubbles in basaltic lava become lined with silica-rich groundwater; later quartz crystallizes inward, sometimes with purple amethyst from iron impurities and irradiation. The host cavity is commonly Cenozoic to Mesozoic in age, depending on locality.
Uses & applications
Used as a decorative specimen, display geode, lapidary material, and occasionally in jewelry; it has little industrial value in this form.
Geological facts
Breaking or sawing a geode reveals an interior that may be hollow, quartz-filled, or partially filled with other minerals. Purple color is strongest where iron-bearing quartz formed under suitable redox conditions.
Field identification & locations
Identify it by a rounded, weathered exterior and a crystal-lined interior; quartz scratches glass and shows hexagonal crystal faces. Common sources include Brazil, Uruguay, Mexico, Morocco, and the United States.
