Rare earth metals are making headlines as demand rises for products from smartphones to wind turbines, and as governments seek secure supply.

These metals are rare earth elements such as rhodium, platinum, and ruthenium. The list includes: Californium is the most expensive metal, valued at around $750 million per ounce. Its rarity stems ...

Iron is the most common material on Earth, making up most of the Earth’s inner and outer core. It is a pure metal, mined and then heated to separate it from rock so that it can be used.