If you are attempting to create a natural setting, the rock and stone should look like it actually belongs in the area. With a little imagination, every size and shape of stone can be used. Smaller stones can be raked up from the surface to create a fine texture mulch. They should be applied over a barrier like black plastic or landscape fabric. Stones mulches around trees and shrubs help to protect the trunks from mower damage during the summer months.
Larger rocks make attractive edging along paths, or around irregular shaped beds.
Rock can be set on the surface or slightly into the soil to produce variety. Larger rock can also be used to create raised planter beds which hold back mounds of soil. With these beds, try to keep the shape slightly irregular which helps maintain a more natural appearance to the design. Rigid forms tend to look too artificial when you are attempting to retain a natural appearance in the garden. Raised areas can be planted to flowers and shrubs.