When the denim is washed, pumice stones are added to the wash cycle process to fade the color of the denim and roughen it up a bit. This gives the denim an aged and faded look.
In the 1970s, designers such as Francois and Marithé Girbaud experimented with the technique. From the 1980s, the production of bleached jeans became industrialized to keep up with their demand. See also ‘Pumice Stones’.
Source: Nouvelle de Nîmes Nº 5: The Denim Dictionary