Katrin Zimmermann slid a silver ring onto her index finger. It looked simple, flat-topped and gem-free. Smiling, she twisted it ninety degrees to reveal a protruding spike. “I call it the Subway Ring,” said Zimmermann, who designed the piece. Zimmermann, 51, is the creator of Ex Ovo, a Harlem-based jewelry business straddling the worlds of … Continue reading KATRIN ZIMMERMANN: JEWELRY & GLOBALISM IN HARLEM