Carolyn Keene 
The Winking Ruby Mystery [EPUB ebook] 

In Europe, the sisters discover that Gino’s family has disappeared, driven into hiding by blackmail and threats of mal occhio, the evil eye. Probing deeper into the strange riddle, Jean and Louise become enmeshed in a mystery more frightening than the dungeon in which they are imprisoned. How they extricate themselves and locate a fabulous crimson gem make a pulse-quickening story.