We think you mean the Miraculous Medal, not the “Miraculous Medallion.”
The Miraculous Medal is a based on a directive from the Blessed Mother to St. Catherine Laboure in November 1830 in Paris:
Catherine saw Our Lady standing on a globe, with dazzling rays of light streaming from her outstretched hands. Framing the figure was an inscription: “O Mary, conceived without sin, pray for us who have recourse to thee.” Then Mary spoke to Catherine: “Have a medal struck upon this model. Those who wear it will receive great graces, especially if they wear it around the neck.”
The vision then seemed to turn to show the reverse of the Medal: the letter M surmounted by a cross with a bar at its base; below this monogram, the Sacred Heart of Jesus crowned with thorns, and the Immaculate Heart of Mary pierced with a sword.
With approval of the Catholic Church, the first medals were made in 1832 and were distributed in Paris. Almost immediately the blessings that Mary had promised began to shower down on those who wore her medal, and soon all of France was clamoring for what the people referred to as the “Miraculous Medal.” Use of the Medal spread from country to country and now blankets the world. Everywhere, the Medal is still drawing down from God blessings for body and soul.
For more information on the Miraculous Medal, visit the website of the Central Association of the Miraculous Medal.