Source : Webster's Revised Unabridged Dictionary (1913)

Rhomboidal \Rhom*boid"al\, a. [Cf. F. rhombo["i]dal.]
   Having, or approaching, the shape of a rhomboid.

Source : WordNet®

rhomboidal
     adj : shaped like a rhombus or rhomboid; "rhomboidal shapes" [syn:
            {rhomboid}]