Xenotime is a rare-earth , the major component of which is yttrium orthophosphate (Y P 4). It forms a solid solution series with chernovite-(Y) 4) and therefore may contain trace impurities , as well as silicon dioxide. The rare-earth elements , erbium, as well as metal elements such as thorium (all replacing yttrium) are the expressive secondary components of xenotime. Due to uranium and thorium impurities, some xenotime specimens may be weakly to strongly radioactive , monazite are sometimes grouped with xenotime in the informal "anhydrous phosphates" group. Xenotime is used chiefly as a source of yttrium and heavy lanthanide metals (dysprosium, ytterbium, erbium and gadolinium). Occasionally, gemstones are also cut from the finest xenotime crystals.