The secret of geodes

geode011As a person who holds a bit more knowledge on certain obscure topics than the general population, I’m sometimes considered a specialist, at least in regard to rocks and fossils from Indiana. That being the case, I often entertain visitors who bring in bizarre objects for identification. Among the most common objects to come through the door are geodes. A fashionable Indiana rock, geodes are mostly found in Morgan, Monroe, Brown, Lawrence and Washington Counties. Most natives of those counties could tell a geode from an ordinary rock, but those of us not born and raised in south-central Indiana often don’t know the identity of these unique natural objects. Geodes are often suspected to be a number of things, including petrified human heads and dinosaur eggs. Unfortunately, neither of these is true. However, they are a minor mystery to geologists.

geode02Indiana geodes originated about 350 million years ago, along with rocks that were deposited while Indiana was underwater (think of a briny middle-eastern tidal flat – very salty and very shallow). The prevailing theory is that geodes originated as an under-sea-floor nodule of the mineral anhydrite. Conditions changed and the anhydrite was replaced, through groundwater flow, by other minerals, resulting in the vast quantity of geodes that we see in only certain rock layers today. By definition, geodes have a hard chalcedony rind, and can either be hollow, with their walls lined by various crystals, or solid.

Many people collect geodes. In southern Indiana, many a fence row is made up of these hard lumpy spheres. They do have another aesthetic value, though. Crack a geode open and you might just find some spectacular crystals, presented nicely in a bowl.

The museum has a collection on display, but through May, a local geode collector, Bob Harman, has been kind enough to loan us some of his best specimens. These are on display in our first floor natural history gallery.

Peggy Fisherkeller is the curator of geology at the Indiana State Museum.