The Collaborative International Dictionary of English v.0.48:

Marble \Mar"ble\, v. t. [imp. & p. p. Marbled; p. pr. & vb. n.
   Marbling.] [Cf. F. marbrer. See Marble, n.]
   To stain or vein like marble; to variegate in color; as, to
   marble the edges of a book, or the surface of paper.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Marbling \Mar"bling\, n.
   1. The art or practice of variegating in color, in imitation
      of marble.
      [1913 Webster]

   2. An intermixture of fat and lean in meat, giving it a
      marbled appearance.
      [1913 Webster]

   3. pl. (Zool.) Distinct markings resembling the variegations
      of marble, as on birds and insects.
      [1913 Webster]

WordNet (r) 3.0 (2006):

marbling
    n 1: the intermixture of fat and lean in a cut of meat