From Middle Persian [Book Pahlavi needed] (lhk' /rahag/, “vein”), also [Book Pahlavi needed] (lk' /rag/, “vein”) whence Persian رگ (rag, “vein”). It must have meant aside from a blood-vessel a “streak, haze in the air”. Persian رگ ابر (rag-i abr) means “streak of a cloud”. Compare the transferred use in mineralogy رَهْج اَلْغَار (rahj al-ḡār, “realgar”), رَهَج أَصْفَر (rahaj ʔaṣfar, “orpiment”) and Old Armenian երակ (erak, “vein or streak of a metal or mineral”).