How would one build a device to detect the potentials of different types of stone. such as galena crystal?

Question

I know of the existence of these devices in the early 1900's.
They were used to determine whether a particular pebble was likely to be useful as a crystal radio component.
But do they still exist, and what are they called?
Is there a schematic or even a description?

How would one be built, if they are no longer sold?

All help is appreciated, thanks.

Answer 1

Basically the galena crystal was used with a "cat whisker" (a sharp wire probe) to find a spot that provided the characteristics of a diode, as detected by demodulation of a radio signal and consequent sound in the headphones. Galena crystals are still sold, both as mineral specimens and as crystal radio set components. In fact there is a thriving crystal radio culture on the web. (See ref.)
To test a mineral specimen for diode characteristics, you would need to connect an ohmmeter to it, connecting one meter terminal with a large area of it, say through aluminum foil, and the other via a probe at many points on the rest of its area. You would test its conduction in both polarities, looking for a significant difference. Or just get or make a crystal radio and use its cat whisker.