By bursting through tunnels and creating vibrations, xerjs create landslides and micro-quakes where metal ores and minerals are suspected to accumulate. This allows the partial extraction of resources from the ground, but requires huge energy costs. On small planets it is easier to create vibrations, so the production is higher there.
Extraction formulas:
1000 * level * (1.1 to the power of level) / (eighth root of diameter of the planet)
500 * level * (1.1 ^ level) / (eighth root of diameter of the planet)
-9000 * level * (1.1 ^ level)