First mentioned in The Adventures of Superman radio show in June 1943, the material has been featured in a variety of forms and colors in DC Comics publications and other media, including feature films, television series, and novelty items such as toys and trading card sets. The established premise is that Superman and other Kryptonian characters are susceptible to its radiation, which created usage of the term in popular culture as a reference to an individual's perceived weakness, irrespective of its nature. To commemorate the 70th anniversary of Superman, the University of Leicester participated in a public relations exercise and presented the Geological Society with "mock kryptonite", termed krypton difluoride.