Actually it wasn't a miracle. While she was still underage, during the occupation of Greece by Turks, one of their captains saw her and he wanted to take her and make her his bride and Muslim because she was very beautiful. But she tried to hide and escape until he broke into her house and took her by force, then locked her in a dungeon and tortured her to change her faith but she refused and prayed until she died from the harsh torments.
Her best friend had a similar faith...and many other girls.
It might not be known worldwide, but the 400 years of the occupation had been very tormenting for my people especially in the matter of trying to force their faith in us. So in Greece we end up having many saints that died because they were Christians during that period.