Natural ytterbium is a mixture of seven stable isotopes, which altogether are present at concentrations of 3 parts per million . This element is mined in China, the United States, Brazil, and India in form of the minerals monazite , euxenite , and xenotime . The ytterbium concentration is low because it is found only among many other rare earth elements ; moreover, it is among the least abundant. Once extracted and prepared, ytterbium is somewhat hazardous as an eye and skin irritant. The metal is a fire and explosion hazard.