The rare earths are widely distributed throughout the earth's crust in extremely low concentrations. They also occur concentrated in numerous minerals but always as mixtures of all the rare-earths, along with lanthanum, yttrium, thorium and other elements. The amount of a given rare earth varies greatly from mineral to mineral, but the heavy rare earths are usually present in low concentrations.
Holmium is one of the less abundant of the rare earths; however, it has been estimated to be present in the earth's crust to about 12 parts in ten million. It is, therefore, more abundant than antimony, mercury, iodine or bismuth. The more important sources of holmium are the yttrium-rich mineral - xenotime, gadolinite, euxenite and fergusonite, to mention a few. In these minerals, holmium is present in concentrations up to 1%. However, holmium also occurs as a trace impurity in many minerals such as apatite, bastnasite and monazite, where it is present from 0.001% to 0.1%.
Monazite and some of the other rare-earth minerals are processed extensively for cerium, lanthanum, thorium and yttrium, and the by-products of some of these processes are now the principle sources of holmium.