The ceramic earphones are made with a brass diaphragm with a soldered connection. I believe these are more sensitive and much more reliable than the aluminum diaphragm model.

High quality, high impedance ceramic earphones for your crystal radio set. The Ceramic Earphones ( Piezoelectric Earphones ) work great in Crystal Radio sets ( Xtal Radio Sets).  The Ceramic Earphone is a high impedance earphone. The Ceramic Earphone's resistance is greater than 20 million ohms and its 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 piezo earphones, piezoelectric earphones, crystal earphones or crystal radio earpieces.

Instead of having a coil in them, Ceramic Earphones 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 or Crystal Earphones. The Ceramic Earphones makes excellent Crystal Radio Earphones.

If you decide to make your own Crystal  Radio Set ( Xtal Radio Set ), remember that radio waves are all that drive the Ceramic Earphone. A good antenna should be used between 50 and 100 feet. If you live near a transmitter tower, then a short piece of wire 15 to 25 feet long will do for an antenna.

Ceramic Earphones are also good for other electronic devices other than crystal radio sets ( Xtal Radio Sets ) due to their high impedance. The high impedance of the 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 operated 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.