The jeweler Paolo Costagli has assembled a mouthwatering collection of unusual, hard-to-find colored gemstones for his new Bespoke series of one-of-a-kind pieces. Extraordinary gem specimens include an intense Windex-blue, cushion-cut 9-carat Paraíba tourmaline in a pavé ring of diamonds and Paraíba tourmalines, and a bright 5.58-carat oval purple-pink sapphire set in a striking ring with diamonds and Paraíba-type tourmalines. Trained as a gemologist, Costagli began his career some 20 years ago working in emer…