Complex sentence: Reynolds's main contribution to the building's design was his insistence that it have a metallic crown, despite Van Alen's initial opposition; the metal-and-crystal crown would have looked like "a jeweled sphere" at night.
Split into simpler sentences: Reynolds's main contribution to the building's design was his insistence that it have a metallic crown, despite the initial opposition of Van Alen. The metal-and-crystal crown would have looked like "a jeweled sphere" at night.