Geodes can be found mainly in North and South America and in Australia.
On the surface, geodes look like an average rock or stone. But crack one open and you are likely to find a hollow cavity filled with crystals. Scientists are unsure exactly how geodes were formed but speculation has centered around three hypotheses. The name geode is from a Latin word meaning "earthlike" in reference to the stone's rounded shape.
Facts
Crystals found inside geodes can be microscopic in size or easily visible to the naked eye. When crystals are so tiny they cannot be seen by the unaided eye, the geode is called chalcedony. Geodes can vary in size from a small as a garden pea to over 30 inches across. Most geodes fall in the 2 to 7 inch range.
Identification
Rounded or oval stones with an exterior of nodules may be geodes. The nodules can have a cauliflower-like appearance. Sometimes, small quartz crystals on the outside of the stone will reflect light and appear to glitter. Since geodes can have a nearly hollow interior, they may feel lighter or seem less dense than other stones their size. The only way to know for certain is to crack open the rock with a hammer to see if it has the geode's characteristic, crystal-lined hollow interior.
Composition
Geodes are composed mainly of quartz. However, most geodes have a band of agate around the hollow area that produces a beautiful color when polished. Other metals and minerals like iron, pyrite, gypsum and calcites are also found in geodes. Each of these colors the interior crystals or the surrounding stone differently, producing scores of geode variations.
Speculation
Geodes have perplexed geologists as to how they are formed. Speculative theories usually have three major stages of formation in common: 1) Concretions, nodules with outward growth, collect around a nucleus and form in soft sediment. These are precursor stones to geodes. 2) Over time, the outer shells of concretions are entirely replaced by chalcedony. 3) The interiors dissolve, creating a hollow space where crystals can grow.
Geography
Geodes are found in North and South America and in Australia. An unusually high concentration of these stones is found within a 35 mile radius of Keokuk, Iowa. The town is referred to as the "Geode Capital of the World" and the state Legislature named geodes the Iowa state rock. Geologists speculate that geodes in this area were part of the Warsaw Formation, a widespread rock strata known to contain these stones.
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