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 . Best Field Indicators are crystal habit, lack of good cleavage, streak, density, pitchy luster when opaque, hardness and color.