I love creating rocks in my watercolor landscapes. Here is how I do it:
1. Start with a mixture of thick, sticky paint, it doesn't even drip when tilting the palette vertical. I mix a combo of ultramarine blue and burnt sienna. Blended in the middle to make a deep, dark rich black. I do not totally mix the color on the palette, I want variety of color in my painting.
2. I wet the top 2/3 of the paper with a pale ultramarine blue.
3. While the sky blue is still damp, I add in greens and browns to simulate the trees. Below the tree area, I add in my thick mix of grays and browns.
4. Using a palette knife with a long diamond shape blade, I carve in the rocks. I hold the blade between my fingers and drag the paint towards myself. The flat edge of the blade rests on the paper, tilting the blade between a 30 and 45 degree angle, sometimes steeper, sometimes not so steep.
5. Twist and turn the blade slightly to form the rocks. Remember to shape your rocks so they are not all exactly the same size and shape. Occasionally clean the knife on a tissue.
6. I like to drag the paint slightly below the painted line to a ragged edge across the bottom.
Give it a try and show me your results!