You need to purchase a marble tile that does not have a built-in spacer or a beveled edge. A "zero grout line is a great way to make 12” x 12” tile look like a solid surface. If your tile has a beveled edge, you will end up with a shadow affect. There is no extra effort involved in this method. Actually, you will save yourself some grout time. One last thing, use your thin-set/mastic sparingly so that it does not squeeze up between your tiles when you set them.