I make this suggestion on the assumption that clay, granite, slate, and marble can appear below the surface without having to be on the surface. If this is not the case then my first suggestion is that it needs to be the case.
Once the above is possible, the prospecting skills should be modified as follows:
Digging at 0: Can prospect for Clay
Digging at 30: Can prospect for Copper
Digging at 60: Can prospect for Iron
Precious Prospecting at 0: Can prospect for Marble
Precious Prospecting at 30: Can prospect for Granite
Precious Prospecting at 60: Can prospect for Slate, Silver
Precious Prospecting at 90: Can prospect for Gold
This way folks can actually use Precious Prospecting in a meaningful way rather than grind it to 60 just to find Silver. Plus, it allows them to find Clay, Granite, Marble, and Slate underground. Adding those resources to the game in underground cells would also make sense as even locations out on the prairie may have valued resources deep underground.