Which tool is primarily used in masonry work for finishing and shaping joints?

The tool used primarily in masonry work for finishing and shaping joints is the brick jointer. This tool is specifically designed to create a clean and uniform joint between bricks, which enhances the overall appearance and durability of the masonry. The brick jointer has a pointed end that allows masons to press into the mortar joint, shaping it to the desired profile. This shaping process not only improves aesthetics but also helps to protect the joint from weathering.

Other tools mentioned have different functions: the pointing trowel is used mainly for applying mortar or working on smaller jobs, while the mason's hammer is intended for cutting and shaping bricks. The margin trowel is typically used for spreading mortar in tight spaces or finishing edges, but it doesn’t specifically shape or finish joints like the brick jointer does. Therefore, the brick jointer stands out as the specialized tool for this specific task in masonry work.