Each one is made using pieces of metalized glass that have been glued together and inclined at a 51-degree angle. It's this angle of reflection, I learned from the article's author, Tomas Rosputinsky, https://www.diybricksx.com/product-category/steampunk-minifigure/ that allows for a secondary rainbow. You can see a colorful effect in the https://www.sembosale.com/ photo above. According to the https://www.diyblocksx.com/product-category/gun-series/ story, Prokop and Broz have to make around 150 to 200 crystals in order to yield one that is perfect enough to install in a final watch.