The best types of rocks for use in rock gardening are those that occur in nature, especially in the gardener's local area. Weathered or aged rocks work better in a rock garden than newly cut stones fresh from a quarry. Harder, more durable rocks such as granite and schist make better choices than soft, porous rocks such as limestone and shale.
If a gardener is constructing a rock garden in eastern California or western Nevada, the most common rock by far is granite. Using large pieces of granite in the rock garden gives a sense of visual continuity to the garden while helping it blend in with the surrounding landscape. Also, chances are large pieces of granite are readily available in this part of the country.
However, if the gardener lives in the Southwest and wants to build a rock garden, the stone of choice is sandstone or arkose. Sandstone is primarily tan or orange, while arkose is typically brown with small, pink inclusions of a mineral called feldspar. While softer than granite, sandstone holds up to weathering fairly well. Metamorphic sandstone, called quartzite, also makes an attractive garden rock, as it has the same color as sandstone while being a much harder, more durable stone.