The Collaborative International Dictionary of English v.0.48:

Rareness \Rare"ness\, n.
   The state or quality of being rare.
   [1913 Webster]

         And let the rareness the small gift commend. --Dryden.
   [1913 Webster]

WordNet (r) 3.0 (2006):

rareness
    n 1: noteworthy scarcity [syn: rarity, rareness,
         infrequency]