When excavating rocks that contain precious metal ores or precious stones, the rate of excavation is determined by the results of a dice roll made using ST + Mining (including relevant excavation specialisms).
A dice roll result of zero or below indicates no progress at all. A dice roll result of 4 produces a normal person's productivity (which will vary substantially depending on what is being mined and the conditions in the mine). The productivity of the excavation increases by 20% per extra point achieved (roughly doubling for every 4 points of dice roll result).