Quick rock tutorial to explain something to someone who asked. Again, I'll do a better one when I improve.
Pencil Tool - A hard-edged brush.
Watercolor Tool - A soft-edged blending brush. You can play around with the settings on this (blending, dilution, persistence) and see what works best for you. Play around with this a lot before using it so you can get a feel for what it does and how it works!
Step #1 - I make the basic shape of the rock with the Base Color using the pencil tool.
Step #2 - Click the little box that says 'Preserve Opacity' above the layers. Then, using a Darker Color, make the general shape of the rock's shadows using the pencil tool.
Step #3 - Switch over to the watercolor tool. Using the same Darker Color, lightly blend the rock's shadows. It helps if you start in the Darker Color and brush outwards (like a real watercolor brush, it should blend nicely into the new color). You can use a larger watercolor brush for this, just get the basics down.
Step #4 - Still using the watercolor tool, switch back to the Base Color and tidy up your blending a little.
Step #5 - Now, decrease the size of your watercolor brush, and make several smaller lines in the rock to add more detail.
Step #6 - Using the same techniques from Steps #2 and #3, make darker shadows at the back of the rock using an Even Darker Color.
Step #7 - I made sloppy edges, so I needed to go back and erase some of the edges to better define the rock. Be sure to unclick the 'Preserve Opacity' box before you start to erase!
Step #8 - Again, using the same techniques from Steps #2 and #3, take the Base Color and add some backlighting to the back of the rock.
Step #9 - Taking a Lighter Color, use the pencil tool (small size) to draw some highlights to give the rock a bit more definition. This is an optional step.
Step #10 - Take the watercolor tool and blend the highlights in a bit more, then use the pencil tool to add some speckles to the rock.
You are done! This is how you get a quick lazy rock.