Go down to the last floor of the mine and dig it yourself if there is no pit. When you come to a seam with a lot of minerals, carefully look for small piles of four minerals or pigments, usually three elbow height. If you dig deep, you have a 50% chance of finding diamonds and mithril, with diamonds and mithril accounting for 2% respectively.
Fifteen, these two minerals are wrapped by other minerals or pigments, and will never appear in the outer layer of the mine.