Make rings two days in advance. Tint 6 oz. fondant gray. Knead 1/2 teaspoon of Gum-Tex into gray fondant.
For rings, use gray fondant jewelry fondant and gum paste mold to mold mesh, chain link or square chain border to make 24 bands. Unmold according to package directions. For mesh border, use ruler and fondant trimmer to cut only two rows of dots. For each ring, use trimmer to cut, 2 3/4 in. long.
Use cornstarch-dusted dowel rod to wrap rings around. Use lightly dampened brush to secure ends together. Lightly press together and flatten slightly where gem will sit. Keep flat to dry overnight.
For gem backs, use gray fondant and jewelry fondant and gum paste mold to make eight of each ring setting. Unmold according to package directions. Let dry overnight.
For gems, tint 3/8 in. ball of fondant each kelly green, royal blue, red, sky blue, rose, topaz, violet and peridot. Reserve some white. Use jewelry fondant and gum paste mold to make eight each round, square and gems. Unmold according to package directions. Let dry overnight.