A stone is a piece of rock. It is a mass of hard, compacted mineral. The word "stone" also refers to natural rock as a material, especially a building material. Natural stone used as building material include granite, marble and sandstone. Manufactured, artificial products, such as concrete or clay bricks, are not stone.
Stone was one of the first materials used to make tools and buildings. It is a very sturdy material. It is less affected by weather than wood or brick. Depending on the type of rock, stone weathers away much more slowly. A stone in the river is reshaped by the water and sediment flowing around it.