The ornamental design for a gemstone, as shown and described.
FIG. 1 is a top plan view of a gemstone in accordance with my new design,
the dashed lines 4--4 not comprising part of the design and only being
used to indicate a surface thereof;
FIG. 2 is a bottom plan view thereof;
FIG. 3 is a front elevational view thereof, the left side elevational view
being a mirror image thereof; and,
FIG. 4 is an elevational view taken in a direction perpendicular to the
dashed line 4--4.