This pumice specimen is from the Oakland Hills in northern California and reflects the high-silica (felsic) magmas that form when subducted marine crust mixes with granitic continental crust. Pumice may look solid, but it's full of small pores and spaces and weighs very little. Pumice is easily crushed and used for abrasive grit or soil amendments.
Pumice is much like scoria in that both are frothy, lightweight volcanic rocks, but the bubbles in pumice are small and regular and its composition is more felsic than scoria's. Also, pumice is generally glassy whereas scoria is a more typical lava with microscopic crystals.
For photos of related rocks, see the volcanic rocks gallery.
Other galleries:
Fossils
Geologic Features and Processes
Glaciers and Ice
Landforms
Minerals
Rocks
Geology and Society