This is based on their composition and texture. Composition Silica amount determines viscosity, violence of eruptions and also the particular rock that is formed. Using the minerals, one can identify the rocks but usually most extrusive rocks are fine grained and use of microscope would be useful. Andesite - gray to green. Grain size - Rocks most important textured characteristic.
Size is determined by rate of cooling and viscosity.
Phenocrysts - large crystals. Tuff, Volcanic Breccia. Textures due to trapped gases. Laboratory experiments 3. Presence of Country Rock - forcing of intrusive igneous rock into the country rock.