According to “Myths & Legends of Cornwall” (C.Weatherhill & P.Devereux), the mines on Rosewall Hill were haunted by Knockers (weird sprite-like things). A miner called Trenwith formed a partnership with the Knockers, and benefitted from their expertise at mining ore. Trenwith would leave 1/10th of properly dressed ore as payment to the Knockers. This relationship continued after Trenwith’s death via his son, who one day sought to cheat the Knockers. The lode then failed, he became a drunk who lost all his father’s money and ended up a beggar.
Sounds like a lovely metaphor for earth-lights, respect for the land and other weirdness to me.