My name is Mike Walters and I'm an amateur geologist. I often come across geodes. A geode can form in any cavity within a rock, but the most common method for geode formation is via gas bubbles that form in cooling ash beds.
The bubble gets "frozen" in place when the silica rich ash / dust hardens. The partial or complete filling of the bubble may occur immediately or even thousands to millions of years later and can encompass a variety of different minerals.
I have opened thousands of these and have found beautiful crystal and mineral deposits. However I have never found anything like this: it appears to be polished metal of manmade origin (close-up above, full image below).
We use a special cutting disc to open the stones. I noticed something odd with the stone; the cutting disc would not cut through, I had to cut circumferentially around the geode to get it opened and I found this object. I used sulfuric acid to dissolve the remaining minerals to free the object. Unfortunately the remaining geode was dissolved, however the metal object is intact. I plan to have a doctor friend take some x-rays of the object.