This cross-section of a polished geode or Thunderegg was photographed with polarized light. This specimen comes from Brazil. My wife and I collect these and often give them as gifts to our children and grandchildren. (See the series below.)
Thunder eggs, or geodes, are geological rock formations which occur in sedimentary and certain volcanic rocks. They look like regular rocks from the outside, the exterior typically made up of limestone, but then have crystal formations of quartz on the inside. They're referred to as thunder eggs by treasure hunters because they do resemble very large eggs because of their smooth exterior and round shape. The real value of the geodes is their appearance when they are halved and polished. Their crystal formations typically come in a wide variety of colors, from deep purple to pink, blue, and even a silvery crystal color. The formations of the rock itself leave a wavy design in the color, and the crystals on the inside emit a fiery glow. It's often difficult to tell what's going to be inside those geodes until you actually split it open, and usually they require some polishing in order for them to be at their full potential when it comes to shine and brilliance. Geodes are common in some formations in the United States (mainly in Indiana, Iowa, Missouri, Kentucky, and Utah). My childhood home town of Nyssa, Oregon is nicknamed the 'Thunderegg Capital of the World' because of large deposits of these semi-precious stones in a nearby area called, Succer Creek Canyon. Geodes also are common in Brazil, Namibia, and Mexico.