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]