|Not part of Riordanverse|
Calamitrons are particles that appear after anyone or anything from one universe comes to or is summoned to another universe. They also cover objects and people from other universes. Gustavo Vidón explains that this is because the calamitrons are "particles that get released when the membrane of the universe is damaged."
They do not, however, appear in the “trade” between two universes are equal, such as switching the same person or object.
Gustavo Vidón has spent years studying the effects of calamitrons has has a very minimal understanding of it, however when he and his assistant read a paper they wrote, they use it to increase their understanding of the matter.