Rocks and boulders add to the naturalism of a garden. From a small rock on the edge of a path to large boulders used for a waterfall they all help make a garden beautiful.
Many beautiful lichens and mosses grow on rocks. In the summer you can bring them to life by spraying them with water.
To look natural rocks should be placed so they are buried in the soil not just laying on top of the soil.
That means that more than half of the rock needs to be buried in the soil. So you need to use a much larger rock than you thought. These rocks look like 400-500 pound rocks but are actually near 1000 pounds each.
Sometimes rocks alone can make a beautiful garden.
Here are a few photos of a garden in Japan that is made of just rocks and sand. It is over 500 years old. It is called Ryoanji.