Geologist Matt Joeckel displays a core sample of carbonatite rock containing niobium and rare-earth elements, which was taken from a deposit near Elk Creek, Neb., in early February. (Associated Press)

Geologist Matt Joeckel displays a core sample of carbonatite rock containing niobium and rare-earth elements, which was taken from a deposit near Elk Creek, Neb., in early February. (Associated Press)