There is no historical evidence that she was thrown into a lake with a millstone around her neck because of her faith.
She survived this until she was shot full of arrows. Her entry is justified because Bolsena in Italy has her relics but this does not include the millstone as far as we know.
St CHRISTINA FACTS AND FIGURES
Name : CHRISTINA
Pronunciation : Coming soon
Alternative names : None known
Location : Worldwide Christianity
Gender : Female
Type : saint
In charge of : Unknown at present
God of :
Celebration or Feast Day : July 24th
Birth and Death Dates : ???-???
Good/Evil Rating : Unknown at present
Popularity index : 1137