Prepare your grill for medium-high direct heat. In a small saucepan over medium-high heat, bring 1/4 c. olive oil to a gentle simmer. Add 2 sprigs rosemary and cook, stirring occasionally, 10 minutes. Remove from heat and let sit while you prepare the scallops. Pat scallops dry. Wrap each scallop with a pancetta slice. For best results, wrap pancetta around the flat side of the scallop first, then around the round side. Next, prepare the skewers: On each one, thread 3 scallops, alternating with 3 chunks of bread. (1/2 loaf (1/2 pound) chewy sourdough bread, cut into 1-inch cubes). Divide rosemary oil into two portions. Brush skewers with the first portion, and season with salt and pepper. Lay skewers on the grill grate and cook, turning when pancetta is crispy on the first side and bread is golden brown, 2 to 4 minutes per side. Using a clean brush, dab the finished skewers with the second portion of rosemary oil. Serve immediately.