Which instrument is specifically designed for adjusting and finishing hard materials like amalgam?

The ball burnisher is the correct choice for adjusting and finishing hard materials like amalgam. Its rounded, smooth surface allows dental professionals to polish and burnish the surface of amalgam restorations effectively. This helps to achieve a smooth, aesthetically pleasing finish while also enhancing the longevity of the material by reducing surface irregularities that could lead to plaque accumulation or potential wear.

The other instruments listed serve different purposes. The amalgam well is primarily used for holding amalgam during a procedure, while the round bur is typically utilized for cutting or removing tooth structure rather than finishing surfaces. The football diamond bur, though it can have some finishing capability, is more suitable for shaping and contouring hard materials rather than specifically focusing on the final polishing and burnishing of amalgam surfaces like the ball burnisher does. Thus, the ball burnisher stands out as the ideal tool in this context.