The Collaborative International Dictionary of English v.0.48:

Naturalness \Nat"u*ral*ness\, n.
   The state or quality of being natural; conformity to nature.
   [1913 Webster]

WordNet (r) 3.0 (2006):

naturalness
    n 1: the quality of being natural or based on natural
         principles; "he accepted the naturalness of death"; "the
         spontaneous naturalness of his manner" [ant:
         unnaturalness]
    2: the quality of innocent naivete [syn: artlessness,
       innocence, ingenuousness, naturalness]
    3: the likeness of a representation to the thing represented;
       "engineers strove to increase the naturalness of recorded
       music"

Moby Thesaurus II by Grady Ward, 1.0:

149 Moby Thesaurus words for "naturalness":
   Atticism, Bohemianism, abandon, absolute realism, affability,
   appropriateness, artlessness, austerity, authenticity, baldness,
   bareness, bona fideness, candor, casualness, chasteness, chastity,
   clarity, classicalism, classicism, clearness, comeliness,
   common speech, common touch, cordiality, correctness, dignity,
   directness, discrimination, distinction, ease, easiness,
   easygoingness, elegance, elegancy, familiarity, felicitousness,
   felicity, finish, fittingness, flow, flowing periods, fluency,
   folksiness, frankness, genuineness, good taste, grace,
   gracefulness, gracility, graciousness, health, homeliness,
   homespun, homeyness, honesty, household words, inartificiality,
   informality, informalness, intactness, irregularity, leanness,
   legitimacy, lifelikeness, limpidity, literalism, literality,
   literalness, looseness, lucidity, matter-of-factness, natural man,
   natural state, naturalism, nature, naturism, neatness, normalcy,
   normality, normalness, offhandedness, openness, order, pellucidity,
   perspicuity, photographic realism, plain English, plain speaking,
   plain speech, plain style, plain words, plainness, polish,
   pristineness, propriety, prosaicness, prosiness, purity, realism,
   realness, refinement, regularity, relaxedness, restrainedness,
   restraint, rustic style, seemliness, severity, simpleness,
   simplicity, sincerity, smoothness, soberness, sociability,
   spareness, spontaneity, starkness, state of nature,
   straightforwardness, taste, tastefulness, terseness,
   true-to-lifeness, truth to nature, unadorned style, unadornedness,
   unadulteration, unaffectation, unaffectedness, unartificialness,
   unassumingness, unceremoniousness, unconstrainedness, unconstraint,
   unconventionality, undisguise, unfictitiousness, unimaginativeness,
   unpoeticalness, unpretentiousness, unrestraint, unspeciousness,
   unspoiledness, unspuriousness, unsyntheticness, verisimilitude,
   vernacular, virginity, wholesomeness