Whether they’re suddenly sexy opaque stones or faceted favorites in unusual hues, these are the five gems to watch in 2018. Fill your pockets accordingly.   Designer Diane von Furstenberg has said that “all fashion begins in the street,” then works its way up to the runways. While the world of jewelry, where trends are slow to emerge and stick, isn’t seeing the exact same phenomenon, something similar is happening: Consumer interest in certain colored gemstones is being dictated by the small, artisan workshops of young urban designers. Certainly the gems and color stories favored by more aspirational names—think David Yurman and Erica Courtney—or red-carpet mainstays, such as Irene Neuwirth and Lorraine Schwartz, are influential. But when emerging designers share their work on social media or, just as often, are highlighted in the press, they create widespread demand