Ornust did a pretty good example of showing how the lighting would be, personally, I think the rock is just too plain, I mean, don't you think it's been out in the wild for awhile or whatever? Here's an edit of mine:

Give it some moss, some bugs, give it some nature man, add some cracks, add some shadows infront of it so it seems like it's in a forest or something(Not exactly sure what I made for that, I guess it's some part of a tree?) . With all this, you get to have a better end product, not only is the rock now being lighted but so are the moss, the bugs on the rock are moving and altogether it'll just feel more real. Also, clean your lines up mah boi.