``A diamond is forever,'' but DeBeers has decided its relationship with N.W. Ayer & Partners, New York, isn't. DeBeers said it will move its U.S. advertising account Jan. 1, 1996, from Ayer to J. Walter Thompson USA. Ayer has had the $35 million account for 57 years, and created the ``forever'' slogan used in DeBeers' advertising worldwide. The switch will make JWT
DeBeers' sole agency worldwide.