I claim the ornamental design for a diamond, as shown and described.
FIG. 1 is a perspective view of a diamond;
FIG. 2 is a right-side elevational view thereof;
FIG. 3 is a left-side elevational view thereof;
FIG. 4 is a front elevational view thereof;
FIG. 5 is a rear elevational view thereof;
FIG. 6 is a top plan view thereof; and,
FIG. 7 is a bottom plan view thereof.
The broken lines shown in the drawings are included for purposes of illustrating environment and form no part of the claimed design. The surface shading lines show contour and/or distinguish between open and solid areas. The surface shading lines are not surface ornamentation.