Graduate student I was working with left
Speak to the professor again. There're a lot of reasons for this:


He set you up with the graduate student, so he can set you up with another graduate student if necessary.
If he set you up with his graduate student, then in some way he's also responsible for you. He won't leave you to your own devices now.
He's still working in the field, so he will know what to do with your idea. He'll probably know what to do with your idea better than the graduate student.
It sounds like you only have an idea. Ideas aren't usually publishable; you need results. Because you're an undergraduate, chances are you won't be able to get those results easily without guidance. The professor can provide that guidance.