The Collaborative International Dictionary of English v.0.48:

Graveness \Grave"ness\, n.
   The quality of being grave.
   [1913 Webster]

         His sables and his weeds,
         Importing health and graveness.          --Shak.
   [1913 Webster]

WordNet (r) 3.0 (2006):

graveness
    n 1: a manner that is serious and solemn [syn: graveness,
         gravity, sobriety, soberness, somberness,
         sombreness]