Traditionally, one would have used galena (natural lead II sulfide) crystals. I also recall most "boys'" radios - the type you'd find in Cubs/Scouts/etc. handbo

Traditionally, one would have used galena (natural lead II sulfide) crystals. I also recall most "boys'" radios - the type you'd find in Cubs/Scouts/etc. handbooks - using a razor blade as the contact whisker, probably because it was the simplest way to get a small contact point and stiffness at the same time with materials a kid would likely have to hand.