Excessive use of rock and statues around a bonsai is known as Saikei, but a single one or two rocks along with the tree still classify as a bonsai. The choice of what type of rock to use basically is up to the bonsai artist. Personally, I use interesting and weathered pieces of granite and slate that I have collected from the wild, but you could basically use anything that looks natural and suits the type of tree for the bonsai, and the pot. If you do use a rock that has been exposed to chemicals or substances that are detrimental to your bonsai - either do not use it, or make sure that these substances are properly eliminated. For your rock 'planting' you obviously need to find a beautiful rock that is harmonious and complimentary to the tree's structure and colours.
Many types of rock exist around the world, but some are better than others for bonsai.
The best kind for bonsai work is a hard type that will not crumble away. It should also have an important colour , shape , and texture . Pay particular attention to the rock's shape and type. It should be intrinsically interesting. A round, smooth rock, for example, suggests a watery scene, so it would enhance trees such as willows.