St. Catherine Labore had a remarkable vision of the Blessed Virgin that would become the basis of the Miraculous Medal.
However, there are certain parts of her vision that are not visible on the medal, such as the jewels that St. Catherine saw on her hand.
Below is St. Catherine's description of the jewels, as was printed in the 1892 book, The Sacramentals of the Holy Catholic Church.
[Mary's] eyes were lifted up to heaven and her countenance was radiant as she offered the globe to Our Lord. Suddenly her fingers were filled with rings and most beautiful precious stones the rays gleaming forth and reflecting on all sides enveloped her in such dazzling light that I could see neither her feet nor her robe. The stones were of different sizes and the rays emanating from them were more or less brilliant in proportion to the size.
Light and dark jewels
According to Fr. Michael Gaitley, some of the jewels were bright and some were dark. He explained in his book, 33 Days to Morning Glory, "Mary explained the meaning of the medal to Catherine as follows: Mary is Queen of heaven and earth. She crushes Satan, who is helpless before her, under her foot (see Gen 3:15). Her arms are open, and the many rays of light are graces that she obtains for those who request them. The dark jewels, the ones that are not full of light, represent the graces that are available but that people don't receive because they don't ask for them."
This symbolism doesn't always come through in the Miraculous Medal, as the jewels on her hand are not visible, or are entirely grey in color.
It appears that Mary was trying to communicate in her vision the reality that God wants to lavish upon us many graces, but we don't always ask for those graces.
God wants to help us, but he stands at the door and knocks. We need to open the door to his grace and let him work in our lives.