When designing a faux rockscape, shape is paramount! Whether it is sandstone or decomposing granite, really take the time and study the common traits found within it's geological family. This will pay huge dividends! If you do not have a reference library, it is very easy to start one. The best place to start is google images and bing images and keep a good filing system that works for you.
The faux rock's shape above was created from a small rock pebble, hence we increased the scale size dramatically and viola! We have a rock!
Note the underbody off to the left side, it does not protrude from the ground like the right side. This is important as it appears this rock had tumbled down and stopped at it's final resting place.
Next time on a hike or walk, collect some cool small rocks that have some interesting shape to them. There may come a day when you can use them!