atomic number 39 - a silvery metallic element that is common in rare-earth minerals; used in magnesium and aluminum alloysmetal, metallic element - any of several chemical elements that are usually shiny solids that conduct heat or electricity and can be formed into sheets etc. fergusonite - a dark mineral consisting of oxides of yttrium and erbium and tantalum and other minerals gadolinite, ytterbite - a mineral that is a source of rare earths; consists of silicates of iron and beryllium and cerium and yttrium and erbium xenotime - a brown-to-yellow mineral that is a phosphate of yttrium in crystalline form |