I've been learning digital painting. Getting realistic materials is tricky indeed! So far, I've struggled with liquids, metals, glass, and the hardest of all: rocks. It's really hard to get the texture and shadow to feel natural! I'd be thrilled to get a few tips here...
Using a digital brush on its own can be extremely limiting. Digital paint is inherently flat and lifeless. Textured brushes can help, but not much. I would recommend employing natural textures via blend modes. The best blend mode for passing textures from one image to another is overlay. In this way the texture of a photo of a rusty surface may be passed onto a painting of a rock. Essentially painting with textures... or photo-bashing.
