The Minecraft modpack Cuboid Outpost gives you a way of producing a full stack of diamond-like material every second, called Thatldu, for nothing. Literally zero cost except five stacks of the material for a singularity plus a bit extra to build the machine. This is only in the first five or six hours; there are way better machines (and materials?) to build later on.
This is a ridiculous amount of production, but it requires processing. I have to smelt the material to use it.
What I really want is to build a machine that can process the full output as fast as it is created. I'm playing with friends, and still there is no conceivable way we will ever need that much of this material. It would be a complete waste.
I still want to, though. It would be funny, and we could use it as building material instead of concrete. Much more fun and interesting than concrete!