The Collaborative International Dictionary of English v.0.48:

cherrystone \cherrystone\ n.
   1. small quahog larger than a littleneck.

   Syn: cherrystone clam.
        [WordNet 1.5]

   2. a half-grown quahog.

   Syn: cherrystone clam.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

cherrystone
    n 1: small quahog larger than a littleneck; eaten raw or cooked
         as in e.g. clams casino [syn: cherrystone, cherrystone
         clam]
    2: a half-grown quahog [syn: cherrystone, cherrystone clam]