I had a Thomas Salter Science electronics set when I was a kid. One of the projects was making a 'crystal' radio using the diode and the variable capacitor. It actually worked! Unfortunately one of the projects further on in the instruction book demonstrated how wiring a diode the wrong way round blocked the current flow. What they didn't explain is it also blew the diode.
The best or certainly the most interesting design for an xtal set involved a centre-tapped coil with a dual ganged variable cap & 2 diodes for double the output - sweeet. As for the sets I built - I cheated by using Germanium dioades rather than making up my own ones.