I love to collect rocks! A cool way to display them in your garden is to stack them. It's fun to try getting all of the various shapes and sizes to balance.
You may have seen people do this at State or National parks. It is not a good practice to do out in nature. It can promote erosion and it leaves a human mark that should always be avoided. Here is a great little article about why people should avoid creating "cairns" in the wilderness.
Stop the Rock Stacking (High Country News)