It's a battery-cell using ferrite (the rock) and aluminium foil as electrodes and his hand as electrolyte. This produces ~0.7v which he then steps up using some

It's a battery-cell using ferrite (the rock) and aluminium foil as electrodes and his hand as electrolyte. This produces ~0.7v which he then steps up using something like a joule thief circuit[1]. [1]: https://en.wikipedia.org/wiki/Joule_thief https://en.wikipedia.org/wiki/Joule_thief