The ornamental design for a gemstone, as shown and described.
FIG. 1 is the top view of a gemstone, showing my new design;
FIG. 2 is the top perspective view thereof;
FIG. 3 is the bottom perspective view thereof;
FIG. 4 is the bottom view thereof;
FIG. 5 is the side view thereof;
FIG. 6 is the side view of a second embodiment of my new design;
FIG. 7 is the top perspective view thereof; the bottom perspective view being identical to the bottom perspective view shown in FIG. 3 of the first embodiment; and,
FIG. 8 is the top view thereof, the bottom view being identical to the bottom view shown in FIG. 4 of the first embodiment.
The crown is shown in broken lines in FIGS. 6-8 in second embodiment. The broken lines are for illustrative purposes and form no part of the claimed design.