Rocks and volcanic projections can be classified by analyzing the size of their particles, their texture, and their shape. The granulometry makes it possible to distinguish ashes (from 0 to 2 mm) from lapilli (from 2 to 50 mm) and blocks (from 50 mm). The texture differentiates the slags, characterized by the abundance of cavities separated by partitions, from the compact lavas and the light pumice stones. Finally, the shape is a relevant criterion for the definition of volcanic bombs.