The Rock - Scoria
Scoria is a textural rock type and not a rock that is classified by mineralogy or chemistry. It forms from lava that is rich in volatiles or gases but is less viscous than a pumice forming lava. When the molten rock is rising in the volcanic pipe, gases begin to form and collect and those gases form large bubbles in the lava. The resulting solidified rock is scoria. Although the open spaces in scoria can be large the rock is generally heavier than water, unlike most pumice which can float on water.
Scoria does not have a lot of uses. In fact the name is derived from a term for waste. However it can be used as an interesting decorative stone with some reddish color. Some of the large Easter Island statues called Moai have scoria stone in their designs.