What it communicates: You're my wifey, but I don't know if I'm ready to make you my wife.
What kind of jewelry are we talking here? Threads of delicate gold or tacky purple pendants hung on red, silk cord? Different pieces communicate different things. If you bring her an ugly trinket you picked up at the flea market because you thought it would bring out her eyes, she'll think you're sweet and helpless. Like a puppy that just pissed on the rug, but didn't know better.
Your safest bet is buying a piece from a boutique she shops in; it will show you're thoughtful, but not super serious. She'll also know you have a genuine desire to get her something that she will actually wear. If you get her something from a major jewelry retailer, she might surmise things are getting more serious.
Just don't screw it up by getting her a heart from Tiffany's. What is this, middle school?