OK, they weren't just odd rocks I picked up. They were made out of hypertufa. I started with two Skippy Peanut Butter jars and cut the tops off both and the bottom off one.

Next, I mixed up a batch of hypertufa and molded a small part of it in an odd shape and pressed it to the bottom of the jar with the bottom still attached and around the edge. I covered that with waxed paper so it wouldn't contact another piece and did the same with another piece of hypertufa. Kept doing that until the first jar was full. Then I taped the two jars together and continued to fill to the top of the second.

After the hypertufa was partially cured, I tore apart the plastic form of the jars and carefully separated the pieces so I would be able to put them back together in the same way they were constructed. OK, turns out I wasn't carefull at all. It took a LOT of TRIAL AND ERROR to get it all back together right, after I painted the pieces. For no good reason, I might build a bigger one some day.