So I've been thinking about this, and I think there is a way to sort of do what I mentioned above. If I create some reactions to 'process obsidian' into different types of obsidian chunks which can then be refined into bars of metal it would work out alright.
Say I had a "Process Obsidian" reaction which turned 10 obsidian stones into obsidian 'chunks' which will be classified as gems (mostly so you can't build with them) and will have a very low value. I might have 5 types of chunks, which you get in various amounts from the processing.
Type 1 - Zinc/Tin/Nickel
Type 2 - Lead/Copper/Bismuth
Type 3 - Silver/Iron/Gold
Type 4 - Platinum/Aluminum
Type 5 - Adamantine
Just crushing 10 obsidian would give you 1 low value chunk, type 1. Running the processing twice would give you two type 1 and one type 2 chunk. Etc
This could be accomplished by giving them % completions in the reaction.
The Type 4 and 5 chunks would be rare, so you'd only get one after 8-10 runs of processing obsidian stones.
These 'chunks' would then feed another reaction, which would turn a certain number of them into bars of metal. Say 3 type 1 chunks would net you 2 tin bars, 1 nickel bar, and 1 zinc bar.
Obviously amounts could be adjusted up or down as needed for balance.
This would make obsidian farming profitable, as you are pumping magma out from the earth, cooling it, mining it out, then processing it for trace amounts of material.