New Delhi, May 18 -- Rare earth metals are on the minds of all investors these days, and for good reason. Recent trade tensions have put China's rare-earth monopoly into stark relief. And the rise of physical AI applications, such as robots, promises to accelerate demand for the materials used in everything from iPhones to fighter jets.

That setup has investors pouring billions into non-Chinese rare earth projects. Sifting through JORC or 43-101 reports, which detail mine specs, isn't easy for a mining engineer, let alone investors sitting atop the New York City skyline. Understanding what any upstarts are up against can help, though.

There are just four mines that currently matter worldwide: The Bayan Obo mine, which is in the Inner Mo...