Several people have asked me if the round rocks we have at Roundrock are geodes, and the basic answer is no, but I’ve found that the answer isn’t as straightforward as that when I examine the heart of the matter.
A geode is a hollow, mostly spherical stone. Often geodes will have crystals that have formed on the inside walls of the hollow sphere. Geodes form in a way that is different from the way my round rocks formed. Basically, my round rocks accreted around a center stone — generally blue-green shale fragments — so by definition they could not be hollow since they must have a center to grow around.
That said, note the hollow center of one of my round rocks pictured above. The explanation for this (and I know you’re waiting to see how I wiggle out of this apparent contradiction) is that this round rock had been cracked open (not by me) and the blue-green shale center within has eroded away, leaving a gap where it once sat. Shale is a much softer stone and will erode easily — if it is exposed. A whole round rock does not yield its soft center to the elements, so its shale would not be eroded. Hence a whole round rock would not likely be hollow.
This open round rock presumably offers the same lesson, though a closer examination, which I am sorry to say is not possible with this photo, suggests a more interesting history. The orangeish inside you see has shapes upon it that suggest shell fossils. This may just be a coincidence of shapes, and I am certainly no authority of fossils. It may be that the orangeish stone is some sort of clay that entered the round rock long after it had been broken and exposed. Thus the apparent patterns in the clay may have some more mundane explanation. Or it is possible that the round rock formed around a stone fragment that had originally contained fossils. (I’m speculating on all eight cylinders now.) One of the ways the meteor impact (which eventually caused the growth of these round rocks) has been dated is by noting the age of the rocks that were churned up by the impact. Rocks from deeper in the strata are older, and they can sometimes by identified by fossils they contain. So fossil fragments were added to the soupy mix in which the round rocks eventually grew, and possibly they provided core material. I may be wildly wrong about this though.
Here you can see how large the core stone must have been (in relation to the size of the round rock that formed around it). What can I surmise from this? Could it suggest that this was a young round rock since it didn’t have as much time to form in relation to the size of its core? (By the way, all of the round rocks in these photos are about the size of a large grapefruit.)
From this one could surmise that the round rock formed around an avocado core.
And here you can see an example of what I think explains why so many of the round rocks (though not all) have a small hole or navel in them. Recall this example. My guess is that such rocks did not have sufficient time to completely engulf their core stone before whatever the geological cataclysm was that moved them out of the soupy breccia they formed within and set them loose in the world. Thus their soft centers were exposed and eventually eroded away, leaving a nice cavity for spiders to live in. Still, these would not be geodes since they have an opening, nor would crystals be able to form within since the constant state of chemisty that is required for growing crystals could not be maintained.