Info on Lava Rocks

Pumice is a type of lava rock that's full of bubble holes. The bubbles formed as the lava was blasted out of a volcano, and were trapped as the lava quickly cooled and hardened above ground. In fact pumice cooled so quickly that crystals could not form. Thus Most of the rock between the bubble holes is volcanic glass.
Pumice rocks are igneous rocks, you can see where little pockets of air have been. This rock is so light, that many pumice rocks will actually float in water. Although it will eventually become water logged and sink.  Because this rock is so light, it is used quite often as a decorative landscape stone. Ground to a powder, it is used as an abrasive in polish compounds and in lava soap.