The ornamental design for a gemstone, as shown and described.
FIG. 1 is a top, front, right perspective view of a gemstone showing our new design;
FIG. 2 is a front view thereof;
FIG. 3 is a top view thereof;
FIG. 4 is a bottom view thereof;
FIG. 5 is a front, right side view thereof;
FIG. 6 is a rear view thereof; and,
FIG. 7 is a rear, left side view thereof.
All of the broken lines shown in FIGS. 3-4 represent unclaimed boundaries of the design. The broken lines shown in FIGS. 1-2 and 5-7 that correspond to the broken lines shown in FIGS. 3-4 represent unclaimed boundaries of the design. The remaining broken lines shown in FIGS. 1-2 and 5-7 are for environmental purposes only and form no part of the claimed design.