flagstone -
One of several types of rock easily split and suitable for making flagstones.
Wikipedia
Flagstone, or flag, is a type of generic flat stone, usually used for paving slabs or walkways, patios, fences and roofing. It may also be used for making memorials, headstones, facades and many construction projects. The name derives from Middle English flagge meaning turf, perhaps from Old Norse flaga meaning slab.