2 days in advance: Make rings. Knead 1/8 teaspoon Gum-Tex into 1/4 oz. fondant for each ring. Roll out 1/8 in. thick. Cut ring using wide end of tip 2A; cut opening using wide end of tip 3. For each diamond, use jewelry mold and fondant to make two round gems; attach flat sides together with damp brush. Let dry on cornstarch-dusted board.
Bake and cool cakes using firm-textured batter like pound cake, in silicone mold supported by cookie sheet. Place on cooling grid over cookie sheet; cover with thinned icing. Let set.
Roll a 7/8 in. ball of fondant; flatten and taper sides into a 1 3/8 in. pillow shape. Position on treat. Use knife to cut a 1/8 in. x 1/2. in. slit in fondant to hold ring.
Use jewelry mold to make dotted trim border. Attach around treat using damp brush; trim as needed. Use Pearl Dust/lemon extract mixture to paint gold border, silver ring and white diamond; let dry. Insert ring in pillow, securing with icing. Attach diamond to ring with icing.