The Ceramic Earphone ( Piezoelectric Earphone ) works good in Crystal Radio sets ( Xtal Radio Sets). The Ceramic Earphone is a high impedance earphone. Its resistance is greater than 20 million ohms and there impedance measures around 16,000 to 20,000 Ohms . This is more than enough for Crystal Radio Sets ( Xtal Radio Sets ). These are sometimes called Crystal Earphones or Crystal Radio Earpieces.
These Ceramic Earphones instead of having a coil in them they operate by a Diaphragm coated with a ceramic material that deforms when a voltage is applied to it. This is called the Piezoelectric effect and thus the name Piezoelectric Earphones or more commonly Ceramic Earphones ( Crystal Earphones ). This makes them excellent Crystal Radio Earphones.
If you decide to make your own Crystal Radio Set ( Xtal Radio Set ) remember that radio waves are all that drives the Ceramic Earphone so a good antenna should be used between 50 and 100 ft. If you live near a transmitter tower like I do then a short piece of wire will do for an antenna. So the use of a good Crystal Radio Earphones is necessary.
These Ceramic Earphones are also good for other electronic devices other than Crystal radio sets ( Xtal Radio Sets ) due to its high impedance. The High Impedance Ceramic Earphone will allow you to power the Ceramic Earphone with very little power usage saving on batteries. These types of Ceramic Earphones are becoming hard to find.
Please note if the ceramic earphone or ceramic headphone are not opperated from a transformer you need a resistor across the audio output. Most crystal radios have a resistor in the audio output, if not add a 47K to 100K resistor.