Proto-Indo-European Roots

Root/Stem: *arg'-
Meaning:  white, to shine; silver
Cognates:
Hellenic Greek argos (white), arguros, arguron (silver)
Italic Latin argentum (silver), Oscan aragetud (silver; abl.sg.), Faliscan arcentelom (a silver coin; acc.sg.)
Celtic  Common Celtic *argent- (silver), >
Gaulish argento- (silver), Old Irish arget, argat, Middle Irish airget, Irish & Scottish Gaelic airgead, Welsh ariant, Breton arc'hant, Cornish argant
Indic Sanskrit arjuna- (white, shining), rajata- (silver)
Iranian Avestan @[email protected]- (silver)
Tocharian Tocharian A árki-, B aerkwi- (white)
Anatolian Hittite harki- (white)
Armenian Armenian arcath (silver)
Balkan Phrygian arg- (silver), 
Thracian  arzas (white, shine)
Notes: The Proto-meaning of this root was obviously 'white, shining' which was preserved in Greek, Sanskrit, Tocharic, Thracian and Anatolian. However, already Proto-Indo-Europeans must have discovered silver and took up this word for it: practically all branches denote silver with this very word. 
The only branch which lacks the root, is Germano-Balto-Slavic which used another stem for 'silver': Slavic *sirebro, Baltic *silabr-, *sirabl-, Germanic *silbr-. Its origin is dark - many think it was borrowed from "Pelasgian" (cp. Sibros argureos potamos - a "silver" river in Lycia), some suppose its Indo-Aryan (from kubhros) or Iranian roots.