The rare earths are moderately abundant elements in the earth’s crust that occur in a large number of minerals, typically as carbonates, oxides, phosphates, and silicates. Rare-earth minerals are found in hard rock and placer deposits located throughout the world, with unusually large deposits occurring in a few countries. In most of the deposits, the disposition of the rare earths in the minerals is such that they can be recovered only as a coproduct or a by-product of certain other minerals. In a few deposits, however, the rare earths can be recovered as the primary or main product. Even though the number of rare-earth minerals is large, much of the actual world rare-earth supply comes from only a few of them. About a dozen more rare-earth minerals also occur in deposits that can be processed easily to supplement the existing rare-earth supply sources. The total rare earths contained in the world rare-earth reserves is considered sufficient to meet the foreseeable demand for these elements, far beyond the twenty-first century.