I think you are onto something there. The piezoelectric effect with quartz would definitely be a good primary candidate. And the iron thing would be confusing, because there is more iron and metals in igneous areas than sedimentary. This is what confused me in the first place.
The iron and quart are in loose unconsolidated soil, so I am not certain it would conduct very well.
Water and dissolved minerals create a better path for conduction. I know that there are sometimes electrical discharges associated with earthquakes. (There are some youtube videos about this.)