You can build one for only a few dollars. Building them was a craze in the 1920s, and again in the 1950s, and now it's happening all over again.
Crystal sets are a great tool for teaching, and a superb hobby.
The crystal radio was discovered in 1901. It is still the basis of modern-day radio and communications equipment and lives on in a wide variety of radio systems. Also on online forum, history and projects. The design of headphones and earphones has advanced and cannot pick up the simple crystal signal, which old-style phones had no trouble doing. But the Xtal Set Society now stocks 2000 ohm headphones and crystal earplugs, capacitors and other parts.