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.

5 Responses

  1. I met Mr. Harman twice in Washington county collecting these. Neato.

  2. Hey I have a couple mystery transparent stones, some hard to define fossils, and a rare desert rose inside a geode. Can I bring them by for and have you take a guess at what they are, or what they are from? One is a very large orthopod 6″x 4″ x2″? I live just outside of Indy, and am a member of the museum.

  3. Great article. You may like Carrie Newcomer’s song on Geodes,
    Around here we throw geodes in our garden.
    They’re as common as the rain and corn silk in July.
    Unpretentious browns and grays
    The stain of Indiana clay
    They’re what’s left of shallow seas
    Glacial rock and mystery.
    Inside there shines a secret brightest promise.
    All these things that we find familiar
    Are just miracles clothed in the common place
    You’ll see if you will try
    in the next stranger’s eyes.
    God walks around in muddy boots
    Sometime rags and that’s the truth.

  4. My husbands family has hundreds of acres of private property in Washington County with about a mile of river bead. Geodes flourish but every once in a while we catch people rapping the river beads of the precious rocks. My oldest daughter wants to be a paleontologist when she grows up and it is the perfect place to take her and for her to look for “fossils”. She would stay in the creeks for hours if we let her. She comes across some pretty interesting things now and then.

  5. I am in New York, but a Hoosier, and collected many beautiful geodes from Heltonville. One has a great baryte crystal inside. They are Hoosier treasure.

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