The ornamental design for a decorativerock, as shown and described.
FIG. 1 is a perspective view of a decorative rock showing my new design;
FIG. 2 is a front view thereof;
FIG. 3 is a back view thereof;
FIG. 4 is a side view thereof;
FIG. 5 is an opposite side view thereof;
FIG. 6 is a top view thereof; and,
FIG. 7 is a bottom view thereof.
In FIGS. 1, 2, 3, 4, 5 and 6, the broken lines showing of environmental structure for purposes of illustrating the decorative rock in a condition of use. The broken lines and the structure they depict form no part of the claimed design.