Perhaps the most fascinating of all rock formations, geodes are typically hollow, rounded rocks in which beautiful crystals have formed.
Geodes start wherever there are pockets of air within rocks. Next, groundwater seeps into theses spaces bringing in trace minerals, which then begin to crystalize inside. The crystals grow toward the center of the cavity over millions of years, gradually forming a glittering lining. Usually, the crystals are quartz, calcite or fluorite, but sometimes other minerals are found. They can have amazingly colorful crystals inside, depending on what minerals were present during their formation. The presence of iron produces red or purple colors; titanium creates blues; nickel or chromium creates green crystals; and manganese creates shades of pink.
Geodes typically have a hard outer shell made of chalcedony, or wavy quartz, that protects the crystals inside from erosion. They usually range from 1-2 inches in diameter, though some enormous geodes have been found. In 2000, a geode measuring 26 feet long and 6 feet across was found in an old silver mine in Spain!
Geodes remind us that it’s what is on the inside that counts.
Learn more fascinating facts about geodes and try cracking one open yourself, with our National Geographic Geode Kits!