Set up the rock in your studio. If you are right handed, put the setup in front of you to the left. This way you will not have to turn to look from the rock to your canvas.
Notice the way the rock is lighted. Change the lighting if you want. Direct light will create strong shadows and will mimic a bright sunny day, while a shaded lamp, some distance from the subject, will produce more subtle shading.
Observe the basic, overall coloring of the rock. Notice how the shadows affect the color. Pick out three levels of shading: dark, mid-tone, and light, and mix these colors. Burnt umber, raw sienna, and yellow ochre are good neutral colors to start with, but don't be afraid to add small amounts of green, red, blue and purple to create subtle variations. Add white to lighten the colors to create the mid-tone and light gradation levels.
Use the mid-tone color mixture to completely fill in the shape of the rock to make one solid form. The basic shape is important for the creation a realistic-looking rock.
Paint the side of the rock that is facing the light with the light color mixture. If the rock is rounded, blend the edge of the highlight while the paint is still wet. If the rock has sharp edges, there will be a hard line between the light and dark areas.
Use the dark mixture to paint the shadowed side of the rock. Your rock might be very round or it may have several distinct planes and sharp angles. Work from observation.
Paint the shadow on the table. This will ground the rock. The darkest dark is often found at the point where the rock meets the table.
Add texture to your painting. Scumble paint by dragging your brush across the dried surface of the painting so that the paint is applied sporadically, rather than as a clean stroke. Apply and scrape paint with a pallet knife. You can go back and forth with your light and dark paint mixtures; paint one over the other, and scrape away until you have the texture you want. Texture will make your rock more realistic.