I was hunting for minerals last night, and I was bothered by the massive network of tunnels I was digging to find veins. We know that Dwarves live underground, and are master miners. These people make their entire livelihood from stone. Surely there are some skilled geologists among them?
I'd love to see a geologist profession with a geologist's workshop. A geologist dwarf would go prospecting, looking for hints in the rock formation concerning the possibly presence of gems and minerals. Maybe he could bring mined rocks back to his shop for testing. When he thinks he has found a possible site for something worthwhile, it would be marked on the map.
This would not be a perfect system, of course, since prospecting isn't an exact science. Cheap minerals / gems would be easier to discover then more valuable ones, and there would always be some false positives and missed veins, or predicted locations that are close, but not dead-on. As a geologist increases in skill, the success rate would increase, or more valuable veins would be discovered, or the range he predict would increase.
None of this would change reality, of course - veins either exist or not, and nothing the geologist would do would ever change this. It might not even be more efficient in time, as prospecting in a region might take as long as blindly digging. But it would probably be more efficient in resources and in space.
Mechanically, I'm not sure how this would work. Would the player designate a region for prospecting, or let the dwarf wander on their own? Should it be possible to prospect for one mineral only ("Find me some iron ore!")? What sort of range should an ability like this have? Should this also point the direction to underground water and magma?