  The diamond cone is a combinatorial description for a basis in a
indecomposable module for the nilpotent factor n+ of a semi simple Lie algebra.
After N.J. Wildberger who introduced this notion for sl(3), this description
was achevied by N. Bel Baraka, N.J. Wildberger and D. A. for sl(n) and by B.
Agrebaoui and ourselves for the rank 2 semi-simple Lie algebras. In the present
work, we generalize these constructions to the Lie algebras sp(2n). The
symplectic semi-standard Young tableaux were defined by C. de Concini, they
form a basis for the shape algebra of sp(2n). We introduce here the notion of
symplectic quasi-standard Young tableaux, these tableaux describe the diamond
cone for sp(2n).
