It all depends on how you made the checkers. The easiest way is to just add an edge in a paint program, nothing wrong with that. Is the checkers a box or is it an image mapped to the marble in the shader? I'd have made a box and changed the Mapping Mode to Box/Face (top) and applied the Checkers function in the shader color channel. Then you can change the height of the box to suit your needs. Additionaly, you could "inset" that box into another one that is slightly larger and colored dark gray for extra dimension.