Murinite was a mineral which could be found on Rigel IV.

The Hill People of the Argus River region on Rigel IV used murinite to make the handles of a type of knives which 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.
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