My mothers family worked in tile and marble for many years. She told me a story once about a specific order they recieved and were using to tile a storefront with marble just after WWII. There was one piece right in the front of the building that was too short to fill the gap. Their marble/tile setter was very talented. He broke the piece, spread it out to size, then filled the gaps with a hardening putty and polished it down. The result looked like a vein in the marble which is usually full of veins. Perhaps a thin vein of a compatible color could be added. I don't know what material he used was or how the back of the piece was reinforced. A marble company may be able to help.
Heck of a dame!, November 2008