Lanthanides
Lanthanides (Z = 57 to 71) form the first f-block. Fourteen elements with extraordinarily similar chemical properties, mainly in the +3 state, essential to modern technologies.
Despite their old name of 'rare earths', lanthanides are relatively abundant in the Earth's crust. Their 'rarity' comes from the difficulty of separating them from one another: their ionic radii decrease monotonically (the lanthanide contraction) and their chemistries are nearly indistinguishable.
Strategic applications: the strongest permanent magnets (Nd-Fe-B), refinery catalysts (Ce), display phosphors (Eu, Tb, Y), high-quality optical glasses (La), lasers (Nd, Yb), optical fibre doping (Er).
Geopolitically, world production is concentrated, making these strategic resources for the energy transition and defence.