DIAMONDS may be forever . . . but chocolates aren't. They tend to melt, or be eaten. Lucullus has cunningly designed five diamond-shaped chocolates and decorated cream cakes to sweeten your mother's day. The chocolates are packed in rectangular-shaped boxes or are in small card-format boxes which open out to reveal a heart-shaped window. The sets include a round brilliant, marquise, pear, heart and emerald-shaped chocolate gem. They are made of smooth, rich Swiss premium chocolate and priced at $115 for the box and $49.50 for the heart. And until May 19, Lucullus has teamed with Chow Sang Sang Jewellery Co. For every $100 spent at Lucullus, a customer will be given a $50 cash coupon redeemable on Chow Sang Sang jewellery. Similarly, for every piece of diamond jewellery bought, over the value of $1,500, the customer will receive a Lucullus cash coupon of $30 which can be used for each $100 purchase at Lucullus. Lucullus produces 45,000 cakes a month. This month pastry chefs have focused on Mother's Day, with different sizes of heart-shaped creamy concoctions which feature a smaller heart and the words ''Happy Mother's Day'' ($135), or ''I Love You Mom'' ($165)on them. A rectangular cake ($145) also has terms of endearment on it. They sound almost too good to eat. For something a little more sinful, the Peninsula Boutique and the chocolate shop with its chocolate fountain in the Landmark, and Seibu, have liqueur chocolates from $76 to $125, or gift boxes from $105 to $430.