That is not a new idea, even in the DRC and it has been an abject failure. Conflict gold, diamonds and coltan still flow out of DRC despite efforts to not buy undocumented minerals. In highly corrupt and impoverished places, documentation doesn't mean anything. One bribe of a few hundred dollars and your Congolese conflict mineral is now clean, documented Rwandan mineral.

Efforts, but have companies been made to pay a cost for failing to source "ethically", even when they say "well gee we tried and it's just really hard"? Things tend to get more serious when money is on the line, rather than just PR. Doing it right would clearly take verification, not just documentation, which means requiring access for outside inspectors.

You can't prove that a certain mineral came from an unethical production location - that's the core problem I believe. How would you punish a company for using conflict minerals if no one can figure out if given minerals are conflict minerals?

Force them to figure it out or start impounding goods? Impound their goods if you can't, for any reason, gain access to inspect any part of their declared supply chain for occasional surprise inspections to verify it is as they say it is, until such time as they switch to recently-inspected suppliers or gain you access?

I don't really get how this is much harder or less practical for the state or states implementing the law than most laws governing corporate behavior, and particularly laws governing imports. Yes a law would have to be drafted, passed, and enforced. I don't think any part of that's particularly less reasonable or more unlikely than in the case of any similar law.

In short, you punish companies for either using conflict minerals or if you're unable to determine to a reasonable level of satisfaction that they are not using conflict minerals. Likely they'll want to make sure you can determine such, and will pressure their suppliers to make it happen, in the usual ways markets do things (suppliers that make it easy and do it right and develop a reputation for having few fine-inducing screw-ups can charge more, those that fail miserably at it may just go out of business and be replaced by others who aren't terrible at their job, pretty soon everything's a little more expensive but we have a decent idea of what comes from where)

You're asking the wrong question.

The question isn't "have we completely stopped blood diamonds", the question is "have we lowered the demand for blood diamonds".

If the answer is yes, then the strategy is legitimate and worked. We may have more work to do, but the argument that we must wait for perfect before attempting to make improvements is asinine.