Religious medals have been used around the neck as a sign of religious devotion and for protection from evil and pestilence by both the rich and the poor for centuries. The religious medal was mostly worn by Catholics in antiquity, but now, a religious medal necklace is cherished by all Christians, including Anglicans, Lutherans, etc.
Saint Genevieve, the patron saint of Paris (c. 422–512), was the first documented case of a Catholic medal necklace being worn around her neck. She received the religious medallion from Saint Germanus of Auxerre. He gave her a bronze religious medal or coin portraying a cross to remind her of having consecrated her virginity to the Holy Father.
During the twelfth century, it was a regular practice among Catholics to wear a religious medal necklace as a souvenir of their trips to places of pilgrimage. Giraldus Cambrensis, circa 1180, told how and his companions returned to London with a patron saint medal in honor of St. Thomas hanging round their necks.
The casting of these Catholic medals continued from the 12th Century until the close of the Middle Ages and even later. But in the sixteenth or seventeenth century religious medals became more artistic and were made of bronze and silver.
Catholic medallions with the image of a lamb or Agnus Dei received blessings by the Pope. By the 16th Century, it was common practice to wear a Catholic medallion necklace that had received a papal blessing and been further enriched by indulgences. Holy medals created in Spain had the profile of Jesus on one side, and of Mary on the reverse, resembling a scapular medal, and Pope Pius V began to grant an indulgence to the faithful who wore this kind of Catholic medal on their hats.
During the sixteenth and seventeenth centuries almost every city in Catholic Europe had craftsmen who devoted themselves to creating religious medals for sale.
Since then, apart from the common run of pious saint medals, various religious medallion necklaces were produced. They made Catholic medals for protection from pestilences. Catholic medals were connected with places, confraternities, religious orders, saints, mysteries, or miracles. And, more Catholic saint medals were created for their devotion.
Source> Devotional Medals