The ornamental design for a gemstonering, as shown and described.
FIG. 1 is a front elevational view of a gemstone ring showing my new design;
FIG. 2 is a side elevational view thereof;
FIG. 3 is a top plan view thereof;
FIG. 4 is a bottom plan view thereof; and,
FIG. 5 is a front perspective view thereof.
The broken lines immediately adjacent the shading lines illustrate the bounds of the claim. The remaining broken lines illustrate portions of the gemstone ring. None of the broken lines forms part of the claimed design.