In the crystal detector to the left you can move the whisker and try to find one of the "magic spots". When you find the right spot you will hear music. Later it was found that these spots corresponded to impurities in the crystal. The impurities that are visible in the picture were not as visible in the real crystal. In fact, the task of finding these impurities was rather difficult because they were not visible at all, so you had to search for them by trial-and-error. The fact that you could not see them and not understand how they functioned made them "magical". There was however no magic involed in the process of rectification, it was only a physical property of the crystal - or to be more precise - a property of the semiconductor the crystal was made of.