The 1N34A Germanium Diode the diode most used in Crystal Radio sets ( Xtal Radio Sets ). 1N34A diodes are made of a material called germanium a material used for transistors and diodes as one of the first semiconductor materials used commercially. 

Crystal Radio Receivers depend on the 1N34A Germanium Diodes rated Forward Voltage Drop ( The amount of voltage before is starts to conduct ) of .3 volts. Silicon diodes are rated around .7 volts. However the voltage for the 1N34A Diode is measured with a current around 10ma. As the current increases so does the Voltage Drop and vise versa . However the 1N34A Germanium Diode actually starts to conduct at very low voltages. The 1N34A will start to conduct around .1 volt. So in a Crystal Radio Receiver you need only to produce around .1 volt from your Crystal Radio Set to begin to hear a station.  That is the reason the 1N34A Diode is so popular with Crystal Radio Set Builders.