The Miraculous Medal comes directly from the Most Blessed Virgin Mary, Mother of God and our Mother; it is a gift from heaven which has never ceased to effect marvels of grace throughout the entire world.
This medal is a very simple and very efficacious means to benefit from the protection of Mary in all our necessities, both spiritual and temporal.
On November 27, 1830, in a residence of the Daughters of Charity, at the Chapel of the Rue du Bac in Paris, the Most Blessed Virgin appeared to Saint Catherine Labouré (1806-1876) for the second time. On this day the Queen of Heaven was seen with a globe under Her feet and holding in Her hands, at the level of the heart, another smaller globe, which She seemed to be offering to Our Lord in a gesture of supplication. Suddenly, Her fingers were covered with rings and beautiful jewels; the rays from these streamed in all directions…
The Blessed Virgin looked down on the humble novice who was contemplating Her. Behold, She said, the symbol of the graces that I bestow on those who ask Me for them. The jewels which remain in the shadows symbolize the graces that one forgets to ask Me for, the Virgin continued. And Catherine Labouré wrote later, She made me understand how generous She is towards persons who pray to Her, how many graces She grants those who ask Her for them, and what joy She has to bestow them! Then there formed around the Mother of God an oval background on which was written in gold letters:
O Mary, conceived without sin,
pray for us who have recourse to Thee.
In a gesture which invited recourse and confidence, the hands of Mary descended and were extended as we see them represented on the medal.
Sister Catherine Labouré beheld this vision with happiness. A voice said to her: Have a medal struck on this model; the persons who will wear it will receive great graces, especially if they wear it around the neck. These graces will be abundant for those who wear it with confidence. The picture seemed to turn around, and Sister Catherine saw, on its reverse side, the letter M surmounted by a little cross, and below it the holy Hearts of Jesus and Mary, the first surrounded by a crown of thorns, and the second transfixed by a sword. Twelve stars surrounded the monogram of Mary and the two holy Hearts.
Sister Catherine faithfully accomplished the mission Heaven had entrusted to her. In 1832 the medal was struck and immediately underwent a extraordinary diffusion throughout the world, accompanied by unceasing prodigies of cures, protection and conversion. Thus it came to be known as the Miraculous Medal. Let us wear this medal of the Most Blessed Virgin with respect, and often repeat with confidence and love, the invocation by which Our Heavenly Mother desires that we implore favors: O Mary, conceived without sin, pray for us who have recourse to Thee.
Source: sanctoral.com