the.com/rare earth

means A group of seventeen metallic elements, used in everything from phone screens to magnets, that are chemically hard to separate from the rock they're found in.

from The 'earth' here is an old chemistry term — before the periodic table, 'earths' were oxide compounds that couldn't be broken down further by the heating methods of the day. The 'rare' stuck because the first ones, discovered in minerals in late-18th-century Sweden, seemed scarce simply because no one yet knew how to tell the elements apart or where else to look — not because the metals themselves were geologically uncommon.