Using a vegetable peeler, remove the zest from the lemon in strips (yellow part only). Reserve the lemon. In a small saucepan, combine the lemon zest with 1/2 cup water and 1 tsp. salt. Bring to a simmer over medium-low heat and cook until the liquid reduces to about 1 Tbs., about 10 minutes. Drain, rinse, drain again, and pat dry. Finely mince the lemon zest and combine it with the grapes, 2 Tbs. of the olive oil, scallions, cilantro, and mint in a medium bowl.
In a small bowl, combine the cumin, paprika, turmeric, cinnamon, and ginger.
Pat the scallops dry. Season them liberally with salt and pepper and coat them with the spice mixture.
Heat 1 Tbs. of the olive oil in a 12-inch nonstick skillet over medium-high heat until shimmering. Add half of the scallops and cook, turning once, until seared on the outside but still translucent in the center, 1 to 2 minutes per side. Transfer to a warm plate. Repeat with the remaining 1 Tbs. oil and scallops.
Divide the scallops among 4 plates and serve with the relish. Cut the reserved lemon into quarters and squeeze over the scallops and relish. Serve immediately.