Waterbottle

Open this photo and stamp out the water bottle.

Zoom in on the waterbottle area.

Click on the Rubberstamp tool. Click on the rock to the right of the waterbottle (not too close) and in Properties set it to really soft and the size to about 10. Make sure "source aligned" is not checked.

Click small areas at a time. Here I'm doing the top of the rock, the flat part.

ALT-click in different places to get different rock shading... 

When you're done, you should have something like this...