Can I only use a crystal earpiece for my crystal radio?
The problem involves the "impedance" concept, so it's usually not discussed in beginners' crystal radio projects.

Crystal radios need a high-resistance earphone.  This is the type that produces tiny sounds when driven by several volts, while only drawing a few hundred microamps.  For AC signals it appears to be a large resistor, 5K or higher.

A standard 8-ohm earphone appears as a much smaller resistor for AC; roughly eight ohms, and it expects a signal of hundreds of millivolts, while drawing a few tens of milliamps.

Yet the RF signal from a crystal radio must be high voltage at low current, so it can well exceed the 0.3V detector voltage.  It's designed to produce few-volts DC output, not few-tenths.  A standard earphone will just short out your receiver, and won't convert very much of the DC output into sound.

There of course is a simple cure.   Connect a small audio transformer to your earphone.   You want to step down the few-volts output by a factor of 20 to 50, converting 8ohms into many Kohms.  Small transformers like this are available, called audio matching transformers, or audio output transformers for old-style transistor radios.  Connect one of these to your tiny earphone, so the high-volt, high-ohms side connects to your crystal radio output.

The above will work, but unfortunately this transformer uses up some energy (as wire heating,) as does the coil in your earphone.  A piezo-crystal earphone is far more efficient than a coil/magnet earphone.

So, for the same milliwatts of EM energy being received by your radio, a crystal earphone will sound distinctly louder than a coil earphone, even if exactly the right audio transformer is being used.  The missing audio ends up as coil-heating.