I claim the ornamental design for a diamond, as shown and described.
FIG. 1 is a front plan view of a diamond showing my new design;
FIG. 2 is a top plan view thereof; and,
FIG. 3 is a bottom plan view thereof.
The rear plan view thereof is symmetrical with the front plan view, and therefore the rear plan view is omitted;
Since the diamond is a circular body, the left plan view and right plan view are identical to the front plan view, the left and right plan views are omitted.