Ask Clay: What the heck is a geode?

Today’s question:

On a visit to Tucson, relatives took me to a gem and mineral show where we saw a collection of geodes. Ordinary on the outside; beautiful on the inside. Will you please tell us how they are formed and where to find them?

I wonder if they are ever going to reopen the state mining and minerals museum at Washington Street and 15th Avenue. They closed it to convert it to a state centennial museum in 2011. Nothing came of that, but we still don’t have the minerals and mining facility back.

That’s a shame because it was a fair to middling nifty place.

Anyway, about geodes:

Geodes form in igneous rock created by cooling lava. Bubbles of carbon dioxide or water vapor form in lava flows and when the lava cools and the gases dissolve an empty space is left.

They also form in limestone or sandstone if some of the sediment dissolves and leaves a hollow space or if a piece of wood or other organic material trapped inside a rock weathers away over time.

In any case you are left with a hollow rock called a pillow. Over the ages, water seeps into the cavity. Minerals in that water form crystals that build up layer after layer as more water seeps in and dissolves.

This can go on for thousands or millions of years. The results are beautiful geodes such as those you saw in Tucson. Geodes are fairly common in Arizona, especially at Diamond Point near Payson and in volcanic fields around Flagstaff.

For reasons I don’t understand, geodes also are common in Iowa, where the geode is the state rock.