She and I knew right away when I met her (that she was the one), so after a year or so of dating, the talk became "when". As such we started shopping together and I let her do her thing. We shopped several places and there was one ring that she lit up when she saw it. So I knew what it was when I went back to purchase it. My decision from there was the stone size and clarity.
So my suggestion, if you're serious with this chick, maybe take her shopping. Yea, it may tell her your intentions, but she will never know the when and where it'll take place. Then, like with mine, pay attention to what she lights up over.
As to the wedding band having/or not having diamonds. There isn't any rule that it has to or not. My Wife's does have diamonds, but yes, the actual wedding band and diamonds are much smaller and generally not the main event, that is the engagement band is for. In many/most cases, they're actually meant to be worn together. So when I got her engagement ring, I also (on my own) had to shop for her wedding band, and I found one that had similar styling to her engagement ring. Usually it's the man's wedding band that is just metal. Mine is anyway.