Outlet covers on new tile backsplash
DO NOT try to chip away the marble no matter how tempting it may be.....most marble is soft enough that you can use a round file to to simply file down the offending edge. You can also use a metal cutting jigsaw blade, held firmly in vise grips, to carefully saw off some marble. Take your time and clear your cutting edge frequently and you can make your cut easily.

Good Luck!