To complement the post I just shared on the Miraculous Medal, here's a bit about St.Catherine Labouré, to whom Our Blessed Mother revealed it.
From Catholic Exchange
Catherine Labouré was a French mystic who initiated devotion to the Miraculous Medal. Born on May 2, 1806, this virgin entered, at an early age, the community of the Daughters of Charity in Paris, France. When she was a 24-year-old novice, the Virgin Mary appeared to her in a vision, seated on the right side of the sanctuary in the community’s mother house. When Catherine approached the heavenly visitor, she was advised by the Blessed Mother how to act in times of trial.
The Blessed Mother then pointed to the altar as the source of all consolation. In another vision of Mary, St. Catherine was told that she would be entrusted with a mission, but that it would bring her great suffering. Our Lady went on to predict the anticlerical revolt that occurred at Paris in 1870. In Catherine’s third vision, Mary requested that a medal be struck showing her as she was then visible to Catherine. This medal has come to be known universally as the “Miraculous Medal.” Mary commissioned St. Catherine not only to have the medal made, but also to spread devotion to it.
St. Catherine did not tell anyone other than her spiritual director and confessor, Fr. Aladel, about these apparitions. It wasn’t until 45 years later that Catherine spoke fully about the visions to one of her superiors. Catherine died on December 31, 1876, and was canonized on July 27, 1947.
The Miraculous Medal, a religious sacramental, is a medal honoring the Immaculate Conception. One side depicts the Blessed Virgin Mary standing on a globe crushing the head of a serpent, which represents Satan as he appeared to Eve in the Garden of Eden. On the reverse side is the letter M, under which are the twin hearts of Jesus (circled by thorns) and Mary (pierced by a sword), circled by twelve stars. Inscribed on the medal are the words, “O Mary, conceived without sin, pray for us who have recourse to thee.” Many miracles have been attributed to the devotion, which accounts for its name.
St. Catherine, I thank you for your part in bringing this sacramental and devotion to the world. Through your willingness to answer our Mother’s call to have this devotion spread, many souls have been blessed. The Miraculous Medal is a powerful conversion tool that imparts countless graces. I thank you, dear Saint Catherine and Immaculate Mother Mary for giving this gift to mankind. Amen.