The "crystal" in the name actually refers to two components both of which are as you say of extraordinary high impedance, about 10M Ohms. The harder part to ge

The "crystal" in the name actually refers to two components both of which are as you say of extraordinary high impedance, about 10M Ohms. The harder part to get your hands on is the "Crystal ear-piece" These were popular in hearing aids circa 1960. They are piezo transducers with a diaphragm in a small housing. Perhaps , as you say, the next best modern component would be a piezo loudspeaker, but check its impedance is several M Ohms. The second "crystal" in the set is a diode. Some designs just use a germanium diode like OA81 , but the authentic design that replicates the POW radios built in camps uses a coiled brass wire and a piece of "coke" (being a form of coal). The coke nugget contains silicon and similar elements. When the springy brass wire touches it just right it forms a semiconductor junction. Getting that to work along with an improvised plate capacitor is pushing luck to the limit, so I am not surprised many kids never get this to work. If anything it's an exercise in tenacity, patience and faith it might be possible. I did actually get this to work for about 10 minutes in 1975 when BBC World Service blasted out on "long wave". Another problem is the disappearance of high power AM transmitters. So you need to be lucky and live in the right place for this to work nowadays.