Flagstone, also called flagging, is a flat stone slab, typically rectangular or square, used for outdoor paving of floors, stairs, pathways, and patios. At Heldeberg, you can purchase bluestone or slate flagstone from 1″ to 2 1/2″ thicknesses. Our stone is classified as hard, fine grained sandstone from the Devonian geological period. It is typically a dark blue-grey stone with a natural cleft face. Irregular flagging is sold by the ton and pattern flag is sold by the square foot.