The element is also found in euxenite and xenotime. Polycrase forms a continuous series with the niobium rich rare earth oxide euxenite. However, thulium also occurs in the minerals monazite, xenotime, and euxenite. These included euxenite, ilmenite, monazite, niobite, and tantalite. Uranium and thorium were in the monazite grains; rare earths were in columbite and euxenite. Euxenite is used as an ore of the rare earth elements it contains.