Wearing human molars as jewelry appears to indicate a certain lack of reverence for human beings: he even refers to the teeth as "specimens." Wolfsheim treats an actual physical piece of a human like nothing more than an interesting object, indicating that he likely views humans themselves as objects. The cuff buttons also seem like a thinly veiled threat, as though Wolfsheim has actually had the molars extracted from someone who crossed him or owed him money or something. The fact that he draws Nick's attention to them out of the blue also seems to support this idea. Wolfsheim says,
"I see you're looking at my cuff buttons." I hadn't been looking at them, but I did now [Nick says]. . . . "Finest specimens of human molars," he informed [Nick].
This is the first time that Wolfsheim has ever met Nick, and, as far as he knows, Nick is a business associate of Gatsby's. It is likely not a coincidence that he points out his cuff buttons to this stranger, as if to say "Don't get in my way or cross me or my next piece of jewelry could be made of your teeth." It's a warning or a signal indicating to Nick what kind of man Wolfsheim is: cold, hard, and ruthless.