Though modding I've sort of done this.
Random rocks can be prospected for metal. Its vastly less efficient than finding that particular metal ore, which is a 1:1 ratio of ore to bars, but random rocks are extremely plentiful, so a 10:1 ratio is still quite profitable and it cleans up the mess while also giving your metal industry a chance to get up and running without relying entirely on ore or goblins.
[REACTION:PROSPECT_ORE]
[NAME:prospect iron]
[SMELTER]
[REAGENT:10:STONE:NO_SUBTYPE:STONE:NO_MATGLOSS]
[PRODUCT:100:1:BAR:NO_SUBTYPE:METAL:IRON]
[FUEL]
One possibility is to use the % for the production in the reaction to allow more random prospecting. For example, going off of
http://en.wikipedia.org/wiki/Earth#Chemical_composition, you could change the product line in this reaction to be:
[PRODUCT:32:1:BAR:NO_SUBTYPE:METAL:IRON]
Thus, for each random stone you process you get 32% of an iron bar. The only problem with this is that most of the metals in the game are trace elements, and that every type of rock has a different composition. I was trying to use the average to reduce the number of reactions I'd have to write, though it would be possible to write a unique prospecting reaction for every type of rock. But again, by going this way aluminum would be one of the most common elements found from rocks (a very large chunk of the Earth's crust is made up of aluminum, but bound in such a way that it requires electricity to efficiently extract), and for most metals it'd be like a 1% gain from rocks, thus requiring 100 rocks to be processed for 1 bar of some metal.
Hmm...actually, on second thought, I might actually get around to try writing up prospecting reactions for all of the rock types. It'd allow the player to selectively remove certain kinds of rock while using other types of rock for building, such as eating up all of the worthless stone for prospecting and saving obsidian for the masons.
And apologies for the rambling nature of this post. I started writing it at 4:40am and insomnia sucks.