Restoration
One of the most durable building materials, stone still does erode eventually, just as mountains are washed to the sea.
These processes are now accelerated through air pollution, which attacks the chemistry of the material.
Other natural erosion processes, such as earthquakes, temperature changes from hot to cold to hot and wind erosion take their toll on stone as well. Water can also have a negative effect by dissolving the cementing components of softer stones and enabling chemical reactions aggressive to the respective material.