Requiring no battery, the crystal radio was one of the earliest forms of radio having been developed in the late 1800s and early 1900s. At this time the crystal radio sets were used to receive Morse code messages, but as time progressed the voice messages could also be received by such sets. This progression had much to do with an improvement in materials, which included the diodes and tuning coil. Even with an improvement in materials though, the construction of a radio set was fairly simple to achieve.
By the 1920s and 1930s radio was taking off, but the sets were expensive objects to buy and so the crystal radio was the cheap alternative that could be built at home. Most major newspapers would run guides on how to build such radios, and it was information that was put to good use during the Second World War. During the war, allied Prisoners of War made use of the materials that they had on hand, to build their own radios, to find out news of the fighting. The soldiers would use recovered wire for the tuning coil and antenna, and make diodes from everyday material, like the pencil lead.
The crystal radio is still used by many people around the world today, although now it is usually a hobby rather than a necessity. In most cases, radio sets are now fairly cheap and mass produced making the building of a crystal radio a pleasure rather than something that needs to be done.