It is a little-known fact that there are actually two categories or types of stigmata: visible and invisible.
Visible stigmata refer to the spontaneous wound marks that appear on a person’s body that correspond to the wounds Christ bore during his Passion—such as the hands, feet, side, brow, and back. Reported cases of stigmata take various forms. Visible stigmatics might bear some or all of these marks. Others only experience the suffering without any outward marks or wounds, and these phenomena are called invisible stigmata.
The most notable visible stigmatic, and the first to be recognized in the history of the Church, was St. Francis of Assisi, whose feast we celebrate today. In August of 1224, two years before his death, he embarked on a journey to Mt. Alvernia in Umbria, near Assisi, to pray and fast. One morning, Francis begged to share in the sufferings of Christ. On the Feast of the Holy Cross (Sept. 14) in 1224, St. Francis had a vision of being embraced by our crucified Lord. The agony of the first Good Friday poured into his being, and he received the the wound marks of Christ in his hands, feet, and side.
One of the better-known examples of an invisible stigmatic was St. Catherine of Sienna, who had mystical experiences and visions from the time she was six years old. In 1375, while visiting Pisa, she attended Mass at the Church of St. Christina. After receiving Holy Communion, she fell into deep meditation, gazing upon the crucifix. Suddenly from the cross came five blood-red rays, which pierced her hands, feet, and side, causing such great pain that she fainted. Here she received the visible stigmata, but through her humility she asked God that they might be made hidden or invisible, and her prayer was heard. Her stigmata remained visible only to her until after her death.