Murinite was a mineral which could be found on Rigel IV. The Hill People of the Argus River region on Rigel IV used murinite for the handles of a kind of knives they produced; one such weapon was involved in a series of murders on Argelius II in 2267. (TOS: "Wolf in the Fold" ) According to the Starfleet Medical Reference Manual, (p.18) the element murinite, symbol Mu, atomic weight 320, atomic number 123, was originally discovered by the ancients on Rigel IV.