They also found rhenium in gadolinite and molybdenite. In particular, thulium occurs in the mineral gadolinite. Some of these have a rare-earth composition similar to that found in xenotime or gadolinite. Niven was looking for the mineral gadolinite, which was used as a street lamp filament. Similar compositions are found in xenotime or gadolinite. Polycrase, or an allied species, was seen implanted upon the gadolinite. It is commonly associated with zircon, monazite, gadolinite, fergusonite, uraninite, yttrialite and pyrochlore. Other minerals known to undergo metamictisation include allanite, gadolinite, ekanite, thorite and titanite. The first rare-earth element discovered was the black mineral "ytterbite" (renamed to gadolinite in 1800). Paradoxically, as noted above, the mineral gadolinite actually contains only traces of this element. This is a mineral similar to gadolinite, "uranotantalum" (now called "samarskite"). During this time, in 1787, he discovered the mineral ytterbite (later renamed gadolinite). The mineral that Gadolin examined was named gadolinite in 1800. However, gadolinite does not contain more than trace amounts of gadolinium. Gadolinite is fairly rare and typically occurs as well-formed crystals.