All Saints have to have two miracles attributed to them--one to achieve beatification, the other for canonization. Mother Guerin became Blessed in 1992. A few years later, the facilities manger for the Sisters of Providence developed glaucoma. He prayed for Mother Guerin's intersession, and touched her tomb. The doctors had told him he would lose his eyes, but suddenly, his eyes were completely healed and cured. I have met this man, shaken his hand and discussed this event with him. To me, it was the most perfect proof of the existence of God I have encountered.
You have made an excellent point. Saints and their relics are miraculous only by the will of God, not of and by themselves. They are intercessory. While the church recognizes them, it does not control them.