You need to convince Gracie after asking Medea to help you cross. You do this simply by having sex with her. You unlock this option after you help Gracie with repairs to her house and after you've helped out Rosie by talking with Gracie about her problem. Once done, go back to Medea, she'll give you the option to explore for the ruby. Get the ruby, go back to Medea again and you'll be able to progress from there.