In Thorvald’s early days, when he was still experimenting with his thaumaturgical abilities, there rarely were situations he could help with. People would be afraid of powers they didn’t understand or others with much more power than Thorvald would solve every problem in the blink of an eye. So he had to train on something small and inconspicuous, shaping it into whatever he felt like.
Years later he still keeps it as a memento, calling it his prism. He has forgotten what it was originally by now after thousands of transformations, but that barely matters.
Some ask him why he still keeps it, which he replies to: “You can find everything in a spectrum, why bring a specific thing if you can bring a prism?”