Rare Earth Elements (REE) are a variety of elements with similar chemistries. Most of them are part of the Lanthanide series of elements. The usual list is: 21 (Scandium), 39 (Yttrium) and elements 57 to 71.
They are important in super magnets, and other electronics. Despite the name, they are not extremely rare being found in trace amounts all over the place. But ores, (where they are naturally concentrated and economically exploitable), ARE rare.
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