Southern Indiana geodes

Look in the creek beds. The rock that held these has already rotted away around them.

Geology.com sets the ground out plainly: in south-central Indiana geodes turn up in exposures of the Harrodsburg Limestone and the Ramp Creek Formation, and the Indiana Geological Survey reports them abundant along streams and scattered over the ground for several miles either side of where those formations outcrop. That is the whole hunting method. Find the formation, then walk the drainages that cut it.

A geode is a secondary formation, which means it did not arrive with the rock but grew inside it later. It is a hollow, roughly spherical body with mineral matter, often crystals, shut inside. They can form in any cavity, but the name is generally kept for the more or less rounded ones. In volcanic rock that cavity is usually a gas bubble, a vesicle in basaltic lava. In the American Midwest, which is what Indiana is, they instead occupy rounded cavities in sedimentary formations, and that is the difference between a geode from a lava flow and one out of an Indiana creek.

The filling is slow. Once the rock around a cavity has hardened, dissolved silicates or carbonates are laid down on the inside surface, and a slow feed of minerals out of groundwater or hydrothermal solutions builds crystals inward into the hollow chamber over a very long time. The colours inside come from whichever minerals and trace elements happened to be present while that was happening, which is why no two open the same.

Then the ground gives them up. The bedrock holding the geodes weathers and decomposes, and anything made of a resistant material such as quartz survives that process and is simply left behind at the surface. The geode outlasts the rock it grew in.

So you are not really digging here. You are picking up the leftovers of a limestone that has already been destroyed, which is why the streams are the place to look.