Proline Decorative Concrete Systems AS300A 24" x 24" Ashlar Roman Slate A Stamp. Proline brings the popular look of slate to the decorative concrete world. The ever-classic Ashlar Stone look dates as far back as the 12th century and is found in most western architecture. We offer a variety of background textures, including this classic Roman Slate. This stamp consists of a 3-part system to allow a more open pattern. Color-coded to reduce repeats, these quality mats both heavy duty and tight fitting for a perfectly balanced fit.