On a recent afternoon in Manhattan, Christian Hemmerle, the fourth-generation member of a family-owned jewelry atelier in Munich, showed me the metallic equivalent of a swatch book: a box of polished rings rendered in 28 different metals. “We start with copper in all its patinas,” he said, before rattling off a list of the other materials—from “very boring” gold to stainless steel in an elegant matte ­finish—that serve as a platform for Hemmerle’s inventive, world-class designs. We sat in a suite at the Hotel Plaza Athenee, a favorite among European ­jewelers for its discrete and luxurious Upper East Side setting. The pieces on display in the sun-drenched space—including a 90 ct. Melo pearl pendant whose beguiling flame-like luster seemed to defy possibility—spoke to Hemmerle’s reputation as a jeweler to the wealthiest and most discerning buyers in the world. Yet Ch