Ytterbium is found with other rare earth elements in several rare minerals. It is most often recovered commercially from monazite sand (0.03% ytterbium). The element is also found in euxenite and xenotime. The main mining areas are China, the United States, Brazil, India, Sri Lanka, and Australia. Reserves of ytterbium are estimated as one million tonnes. Ytterbium is normally difficult to separate from other rare earths, but ion-exchange and solvent extraction techniques developed in the mid- to late 20th century have simplified separation. Compounds of ytterbium are rare and have not yet been well characterized. The abundance of ytterbium in the Earth's crust is about 3 mg/kg.