1. THE PRAYER: “O Mary! conceived without sin, pray for us who have recourse to thee!”. These words surrounded Blessed Mary in St Catherine vision in 1830.
2. THE RAYS: projecting out from Our Lady’s hands, the rays symbolize the graces shed upon those who ask for them.
3. THE GLOBE: Mary standing on the globe crushing a serpent beneath her feet symbolizes her Assumption into Heaven.
4. THE STARS: The twelve stars on the back of the medal symbolize the Twelve Apostles who form the Church.
5. THE M WITH THE CROSS: The M stands for Mary, the Mediatrix who presents our prayers to her son to seek His graces.The Cross and Bar surmounting Mary’s name symbolizes Christ suffering on the cross and redemption. It also reminds of her presence at the foot of the cross at her son’s death.
6. THE TWO HEARTS: The Sacred Heart at the left is adorned with crown of thorns, symbolizing His Passion and Love for us All reminding us that he died for our sins. The Immaculate Heart at the right represents Mary’s heart pierced by a sword. The fire shooting up from each heart symbolizes purity and intensity of love between Mary and Jesus.
(source)

 Get one for free here.

Documented Miracles Attributed to the Miraculous Medal – article