Rocks come in different sizes, shapes and with various kinds of surfaces. Whether it’s an imposing limestone cliff face or a small rock on a river bed, I take the same approach when painting them.
I try to be aware of the overall shape and form of the rock structure, along with its texture, because both of these are what give a rock its character. The overall shape is controlled mainly with larger brushes painting at lower zoom levels, whereas the texture and elements such as cracks are painted with both custom brushes and smaller-sized brushes.
I often use several photos of rocks as reference and study them both in terms of their three-dimensional shape and surface details. Also, be aware of where the light is coming from: the way the light interacts with the rock is what makes it believable. The sunny side has a lot of textural detail. The shadowy side, on the other hand, should be less detailed and receive bounced light from nearby surfaces or from the sky.
The overall shape gives the rock its character, but it’s the textural details that makes it look believable. It’s important that your textural brushstrokes follow the form of the planes of the rock.