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Near the end of the process, Graham adds a layer of corrupted plastic, which gives the geode its rough exterior texture. “I was mixing the urethane with some sand once, and the urethane just bubbled up like baking soda and vinegar. I realized it had to be the moisture, so I began adding a little bit of water. Actually after I talked to expert, he was like, ‘Oh yeah! That’s how you make foam.’ I really love the volcanic look of it.”