Brief explanation:

Scoria, the less silica-rich equivalent of pumice, are vesicular (bubbly) glassy volcanic rock fragments of basaltic to andesitic composition that are ejected during explosive eruptions. Scoria is the common product of strombolian eruptions or lava fountains. The bubbles are due to the escape of volcanic gases from the magma during the eruption when they are 'frozen' inside the glass. Scoria is typically dark gray to black in color, mostly due to its high iron content. Sometimes, oxidation (of iron) leads to a deep reddish-brown color.