Is any ring a promise ring?

You can use any ring as an engagement ring. There are no rules saying that it has to be a diamond ring, or any ring in particular. The only guide line is that the ring be worn on the traditional...

yes if you wont A Purity ring can be whatever you want but most people like the idea of a cross or some kind of inscripting.

I suggest you wear it when you are at least 13. A promise ring is to stay pure (Not have sex) until marriage, and 13 is the appropriate age. :)

There really isn't much of a difference. Most of the time Promise rings are cheaper, and engagement rings are more expensive. A Promise ring is kind of a way of saying someday i will propose to you...

No, it's not even like an engagement ring. It's more like an I want you to be my girlfriend ring.