This ending comes from the PIE adjectival suffix *-no-.
Suffix added to nouns to produce adjectives meaning "made of, of the nature of" (such as golden, oaken, woolen), corresponding to Latin -anus, -inus, Greek -inos; from Proto-Germanic *-ina-, from PIE *-no-, adjectival suffix. Common in Old and Middle English (e.g. fyren "on fire; made of fire;" hunden "of dogs, canine"), the few surviving uses are largely discarded in everyday use, and the simple form of the noun doubles as adjective (gold ring, wool sweater). Some are used in special contexts (brazen, wooden).
Not to be confused with the verbal formation '-en' that forms darken and weaken
Word-forming element making verbs (such as darken, weaken) from adjectives or nouns, from Old English -nian, from Proto-Germanic *-inojan (also source of Old Norse -na), from PIE adjectival suffix *-no-. Most active in Middle English.