Black and White Rock Texture
CC0 Public Domain Image Pack
It is not as easy as it sounds. A couple of pointers, in case you want to follow in the madness.
1) Start drawing circles at different points on the page. I would suggest you draw 10 to 20 circles, then jump over to another spot on the page, draw 10 to 20, then move around. Then slowly start filling in the space in between. This makes the page more uniform looking since you will draw circles differently as your hand gets tired.
2) Sharpen your pencil, but then scribble with it on a different piece of paper until the point gets dull. The difference between circles drawn with a sharp and a dull point is very noticeable when you scan in the paper at 600dpi.
3) Take a break every couple of minutes so your hand doesn't get tired. You will start to draw more unevenly if your hand is tired. I especially had an issue with a stray jagged line at the end of making the circle if I did not concentrate on it.
|low res example of rock texture|
I posted my first version on G+ when Bill Logan suggested I put it up under a "Pay What You Want" price. Then Alex Schroeder mentioned how he always sees these kind of images as tiles, so they can stack up against each other seamlessly. So I went back and started working on a version that would do just that. The below image is composed of 4 of the top image.
|low res example of 4 tiled rock textures|