It is composed of two rather scarce elements, yttrium and beryllium. Yttrium is a rare earth metal and rare earth metals are finding more and more industrial uses. At times significant quantities of other rare earth metals such as cerium, lanthanum and neodymium will replace some of the yttrium. Color includes black, greenish-black or green. Luster is vitreous to pitchy if black.
Crystals are transparent to translucent if green, opaque if black.
Cleavage is indistinct. Fracture is conchoidal. Streak is green to gray-green. Index of refraction equals 1.79. Associated Minerals include xenotime , synchysite , aeschynite , monazite , fluorite , allanite and some feldspars .