When Alexander the Great conquered the Egyptians, the Greeks adopted the tradition of giving rings to thier lovers to represent devotion.  Many of these rings depicted Eros or Cupid, the god of love.  When the Romans conquered Greece, they picked up on this tradition and began using iron and copper rings in marriage ceremonies.  The iron rings sometimes had key motifs to Symbolize that the wife now had control of the household goods.  By 2nd century CE, however, most rings were gold.

 

Ancient Egyptians believed that the ring finger of the left hand. contained a "vena amoris" of "vein love" that led directly to the heart. The Romans adopted this belief and wore wedding rings on their ring finger.  Although their belief isn't anatomically correct, the tradition of wearing rings on the ring finger continues to this day.