Well, it started out by trying to use up leftovers! I had a bunch of chopped up transparent soap from the purple crystals, and grey soap dough from the pebbles, and it struck me that they could be combined into a geode.
I got a bunch of reference pictures together and dug in. Basically I made rectangles of all the different layers I wanted, stacked them on top of each other, spread over the chopped up crystal bits, and rolled the whole thing into a sausage. When it was set enough I sliced it up. Then later on I added the rough edging; it's old soap dough in a few shades of grey chopped up in the food processor and stuck on by the handful.
The second ones I made were a lot better, rather than stacking all the layers at once, I wrapped them one at a time. This gave a lot more control and I didn't have all the joins in the same place. I also spritzed with alcohol in between to help them adhere. When the whole thing was done I squished it gently up and down the length, which helped the layers to stick, made the whole thing thinner and longer, and gave an interesting unevenness.