If your rivet is a shape layer, that white line is the path. If you click on the path or on another layer entirely, the white line will disappear. It isn't even really there, it just gets highlighted when the shape layer is active.

Other than that ... if it's NOT a shap layer or path, I have no idea what that white line is.

As for the embossing, I would set the height/size higher so it's not so flat looking. This is assuming that you're using Layer Effects to get that effect.