A crystal earpiece typically consists of a piezoelectric crystal with metal electrodes attached to either side, glued to a conical plastic or metal foil diaphragm, enclosed in a plastic case.
The plastic earpiece casing confines the sound waves and conducts them efficiently into the ear canal, to the eardrum.
Crystal earpieces are usually monaural devices with very low sound fidelity, but high sensitivity and impedance.
Crystal earpieces can also be used as rudimentary, low voltage, audio circuit troubleshooting tools; it is sufficient to touch the tip of the earpiece's audio connector on a point of interest while simultaneously touching the other (sleeve) connection with one's finger.
This quick-and-dirty technique can remove the need for setting up and probing with an oscilloscope or connecting an amplifier to the test point in the first instance.