Another quick tutorial while I am still reworking the space tutorial. This one is another request – how to create rocks, that don’t look like clouds. 
The common mistake when creating shading is using ‘pillow shading’, where the shading is same in all directions, same amount of darkness and distance to the edge. It will look smooth but unbelievable for anything harder and more edgy than clouds or pillows. 
In this tutorial I use three different styles of shading:
– shading through interpolation, 
– shading with lighter and darker shapes [cartoon shading] and
– shading using transparent gradients
I forgot to use the easiest of all for the rounded rock, which would be a radial gradient. Gradient shading [linear or radial] can be very effective but tends to look off as soon as the shapes become more complex.
Let’s get started with a simple rounded rock, pebble, boulder, whatever you want to call ti. 

For the veins I used the freehand draw tool, with a white stroke that I ended up putting into a clip to match the shape of the pebble.

Another shape commonly used for rocks is a lot more edgy and clear cut. I worked mainly with the straight line tool for this one. You can also start with the square, make it a path and add nodes to it, if you find that approach easier. 

I hope I made enough sense for you to create rocks on your own now and enjoy doing it.