The ornamental design for a tombstone, as shown and described.
FIG. 1 is a front, top perspective view of a tombstone showing our new design;
FIG. 2 is a front view thereof;
FIG. 3 is a rear view thereof;
FIG. 4 is a right side view thereof;
FIG. 5 is a left side view thereof;
FIG. 6 is a top view thereof; and,
FIG. 7 is a bottom view thereof.
The features shown in broken lines are portions of the tombstone forming no part of the claimed design. The features shown in long dash-short dash bolded broken lines indicate the bounds of the claimed design and otherwise form no part of the claimed design. Any portions of otherwise claimed surfaces obscured from view by features shown in broken lines are unclaimed. The left side of the boundary line surrounding the face of the tombstone, which is visible in FIG. 2, is not shown in FIG. 1, as the forwardly extending curvature of the left cheek and forehead face portions away from the left side of the boundary line surrounding the face and toward the center of the face hides the left side of the boundary line surrounding the face from view in FIG. 1.