Proline Decorative Concrete Systems RS160 34" x 36" Random Belgium Slate Concrete Stamp. Prolines Random Belgium Slate gives you stones ranging in size from approx. 8 Inch x 10 Inch to 12 Inch by 18 Inch. Each stone has a lightly hammered edge and defined grout lines for a natural quality. A single tool application and, as with all Prolines patterns, we offer this pattern in a thin-flex mat for finishing against walls and walkway edges. Belgium Slate adorns the background of these stately stones. Consider accenting this piece with any of our many band and border tools.