### Instruction:
What are rare-earth materials

### Input:
A rare-earth mineral contains one or more rare-earth elements as major metal constituents. Rare-earth minerals are usually found in association with alkaline to peralkaline igneous complexes, in pegmatites associated with alkaline magmas and in or associated with carbonatite intrusives. Perovskite mineral phases are common hosts to rare-earth elements within the alkaline complexes. Mantle-derived carbonate melts are also carriers of the rare earths. Hydrothermal deposits associated with alkaline magmatism contain a variety of rare-earth minerals.

### Response:
Although originally thought to be rare, many of the minerals are actually common in the Earth's crust. However, due to the difficulties in extracting the metal from the ore, rare is a fitting term.