The Collaborative International Dictionary of English v.0.48:

Curbstone \Curb"stone`\ (k?rb"st?n`), n.
   A stone set along a margin as a limit and protection, as
   along the edge of a sidewalk next the roadway; an edge stone.
   [1913 Webster]

   Curbstone broker. See under Broker.
      [1913 Webster]

WordNet (r) 3.0 (2006):

curbstone
    n 1: a paving stone forming part of a curb [syn: curbstone,
         kerbstone]

Moby Thesaurus II by Grady Ward, 1.0:

31 Moby Thesaurus words for "curbstone":
   Tarmac, Tarvia, asphalt, bitumen, bituminous macadam, blacktop,
   border stone, brick, cement, cobble, cobblestone, concrete, curb,
   curbing, edgestone, flag, flagging, flagstone, gravel, kerb,
   kerbstone, macadam, pavement, pavestone, paving, paving stone,
   road metal, stone, tarmacadam, tile, washboard