A rare black opal, as large and undistinguished as a yam, rests deep in the earth’s crust.Sheathed in gray sediment since the days of the dinosaurs, the timeless stone waits in silence, over countless millennia, for the moment its magnificence will be acknowledged.Howard Ratner (Adam Sandler), the protagonist of “Uncut Gems,” also awaits his moment. But there is nothing silent about him.A gem dealer in the diamond district of Manhattan, Howard — a loving father, cheating husband and gambling addict  …