flagstone

Etymology

flag + stone

noun

1. A flat, rectangular piece of rock or stone used for paving or roofing.
   Grass sprouted between the pebbles and a greenish stain lay upon the flagstones. 1891, Sir Arthur Quiller-Couch, The Blue Pavilions
   In 1946, several of the heavy flagstones, which form the decking of the viaduct, were replaced or reseated, and manholes were provided in the decking, to facilitate access to the inner walls. 1949 May and June, “Notes and News: First Repairs in a Century”, in Railway Magazine, page 202
2. One of several types of rock easily split and suitable for making flagstones.