Celtic jewels, Celtic culture seems to have been already formed around the third millennium B.C.E. It represented the most important center of population in Europe during the Iron Age, and already classical sources were using the term “Celtic” (from the Greek Ksltoi or Ksltai) to describe a population speaking an Indo-European language. In their period of highest splendor (the IV-III centuries B.C.E.), the Celts were spread across a wide .