Scoria is the basaltic equivalent of pumice. It is a vesicular (bubbly), glassy rock formed during eruptions, when decreasing pressure causes gas to «bubble out» of andesitic and basaltic magma. Scoria is dark gray to black in color, mostly due to its high iron content. Unlike pumice, scoria is usually too dense to float on water.
Scoria from very recent eruptions on Stromboli. Photo: J. Alean