borrowed from German Rhodonit, from Greek rhódon "rose" + German -it-ite entry 1 — more at rhodo-
Neither the formation of the German word nor its origin are entirely clear. A direct derivative from Greek rhódon would be Rhodit, assuming truncation of the inflectional ending, so that -on- can only be explained as either importation from another word or an unusual (for the time) failure to abide by classical word formation rules. Mineralogical references (as mindat.org) credit the word to the German mineralogist Christoph Friedrich Jasche (1781-1871), though its first use appears to be in an article by the mineralogist Ernst Friedrich Germar (1786-1853), "Ueber die kohlenstoff- und kieselsauren Manganerze des Unterharzes," Journal für Chemie und Physik ("Schweiggers Journal"), Band 26 (1819), pp. 108-120. Germar refers both descriptions of the mineral and its name to Jasche's article "Das Rothmanganerz in der Gegend von Elbingerode am Harz" in Jasche's Kleine mineralogische Schriften, Band 1 (Sondershausen, 1817), but Jasche uses neither Rhodonit nor other generic mineral names mentioned by Germar.