Geode-like rock but it's empty inside. Why does this happen?

I found this geode-like rock (spheroidal shaped with empty spheroidal cavity inside) near Dickson Tennessee, USA. Comments suggest that this is in fact geode, but it is empty and featureless inside.

The white on the outer part is soft and can be flaked off.

1. Can this still be a geode? Assuming it is, why might this one be empty inside instead of being full of crystals?
2. Would this be indicative that most geodes in this area are likely to be empty as well, or would there be no correlation between one and another?

Click any image for full-size view, scale is in inches:

• It's a geode. You're unlucky, your geode is empty. Most have pretty quartz crystals inside and are worth a small sum. A few have amethyst or opal inside, and are worth a much larger sum. Where there is one there are usually more, so why not go back and see what you can find? You might be luckier next time. If there is a limestone cliff near were you found it, that might be the place to look , but they can occur in other types of rock. Usually the walls are thinner and the cavity larger. They are very variable in size, but yours is typical. – Michael Walsby Sep 4 '19 at 16:20
• Thank you so much, Michael!!! I was wondering if it might be a geode, but I had never seen one without the crystals... I will definitely go back and look for some more... :) – Kendra Sep 4 '19 at 17:01
• I've modified your question in order to remove the identification-request tag. That tag is being discontinued. Feel free to modify further, and welcome to Stack Exchange! – uhoh Sep 5 '19 at 7:41

1 Answer

Geodes form where there is a gas bubble or some other sort of cavity in volcanic or sedimentary rocks. I once found scores of them eroding out of a sea cliff in Oman, and they looked very similar to yours. I can tell that your geode was formed in sedimentary rock like mine were. What happens is that over millions of years, water seeps in, sometimes bringing in sand and clay particles which line the cavity when it occurs in sedimentary rock. This mineral rich water, sometimes from hydrothermal sources, seeps into the cavity and precipitates various minerals, mostly silicon dioxide, around the sides of the hollow. This usually results in a lining of quartz crystals, but can produce amethyst or opal, both of which are composed of silicon dioxide. Very rarely, opalized fossils are found inside geodes. As for your empty geode, I presume that either the necessary seepage of mineral-laden water never happened, or it never had time to happen. The process takes millions of years.