The various types of igneous rocks, once molten rock, have all since cooled into their current solid forms. Two forms of igneous rocks -- pumice and volcanic scoria -- are known as vesicular types which formed on lava flow surfaces.
Because igneous rocks cooled at different rates, they took on different appearances, like the vesicular (porous) appearance of pumice and scoria. The vesicles are holes created by gas bubbles in the foamy, upper layers of the lava before it cooled and hardened.
Pumice contains small, frothy gas holes, is light in color and, due to its viscosity, will float. It was produced by lava containing a large amount of volatiles (water and gas), large amounts of silica, and very little iron and magnesium. Pumice was used in concrete by the Romans and is used today in construction and landscaping.
Scoria ranges in color from red to black, is heavy due to heavy metal content, and extremely porous with a spongy appearance caused by large gas pockets. The heavy metals cause scoria to sink in water. Scoria is generally considered waste material and is only used for decorative purposes. Some of the Easter Island statues incorporate scoria in their design.