The Collaborative International Dictionary of English v.0.48:

Flagstone \Flag"stone`\, n.
   A flat stone used in paving, or any rock which will split
   into such stones. See Flag, a stone.
   [1913 Webster]

WordNet (r) 3.0 (2006):

flagstone
    n 1: stratified stone that splits into pieces suitable as paving
         stones [syn: flag, flagstone]

Moby Thesaurus II by Grady Ward, 1.0:

49 Moby Thesaurus words for "flagstone":
   Tarmac, Tarvia, adobe, ashlar, asphalt, bitumen,
   bituminous macadam, blacktop, brick, bricks and mortar, cement,
   clinker, cobble, cobblestone, concrete, covering materials, curb,
   curbing, curbstone, edgestone, ferroconcrete, firebrick, flag,
   flagging, flooring, gravel, kerb, kerbstone, lath and plaster,
   macadam, masonry, mortar, pavement, pavestone, paving,
   paving material, paving stone, plasters, prestressed concrete,
   road metal, roofage, roofing, siding, stone, tarmacadam, tile,
   tiling, walling, washboard