The ornamental design for a decorative rocklight, as shown and described.
FIG. 1 is a perspective view of a decorative rock light showing my new design;
FIG. 2 is a front elevational view thereof;
FIG. 3 is a rear elevational view thereof;
FIG. 4 is a left side elevational view thereof;
FIG. 5 is a right side elevational view thereof;
FIG. 6 is a top plan view thereof;
FIG. 7 is a bottom plan view thereof; and,
FIG. 8 is a perspective thereof, showing the decorative rock light in a state of use.
The broken lines in the drawings depict portions of the decorative rock light that form no part of the claimed design, and the broken line showing of the line in FIG. 8 is for environmental purpose only and form no part of the claimed design.