You could say that mountains and stones decompose over many thousands and even millions of years, although the terminology geologists use is that they" erode."
Mountains are made up of rocks (and stones) and stones are made up of minerals. Over time, water erodes the rocks that make up a mountain. It forms streams and rivers that slowly cut through the rocks that form mountains, forming canyons. The rivers carry the rocks and stones off the mountain and into a valley. In this way, over millions of years, mountains that were once tall lose their mass and become smaller.
In a similar way, water breaks the bonds that hold the grains and minerals that make up a rock. These smaller grains break off of the rock and flow in the water as sand, silt or clay. We call these "sediments" Over thousands of years, rocks may lose all of their grains, which flow off as sediments that are deposited elsewhere. These sediments may form the sand you find at the beach, or the clay bank of a river, or the soil that you might find in your neighborhood. If it's not made up of rocks, most of the Earth's ground is made up of sediments that used to be rocks.