Quartz

Unknown Owner

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A matching pair of beautiful large thunder egg agate. This was a large nodule that has been cut in half with one side on each half polished. Thunderegg agates form within silica-rich volcanic rock layers, primarily rhyolite lava flows and welded tuffs. The process begins when cooling volcanic gas bubbles or contracting rock layers leave spherical cavities, which are initially filled by expanding nodules of clay or intergrown cristobalite and feldspar fibers called spherulites. Over time, tectonic or thermal pressures cause these interior nodules to crack and split open into characteristic star-shaped geometric chambers. Silica-bearing hydrothermal fluids then slowly percolate through the porous volcanic host rock and enter these internal hollow chambers. As the fluids evaporate or cool, they deposit successive microscopic layers of chalcedony, quartz, and macrocrystalline varieties against the interior walls, often producing intricate bands of color, moss-like inclusions, or solid quartz centers that remain encased within the weather-resistant rhyolitic outer shell.