You may never think of your innocent little pumice stone the same way once you realize it's a piece of frothy, bubbly lava that was violently ejected from a volcano, then left to cool and harden. If the lava that became a piece of pumice had been left to cool in higher-pressure conditions, it would have become solid obsidian instead.
Porous
Vesicles, the irregular cavities in a pumice stone, are often rounded, elongated or appear as tiny tubes running through the rock. Vesicles occur because the gas that was dissolved in the near-liquid pumice when it was hurled from the volcano, is suddenly released as the rock cools, producing bubble-like cavities in the rock.
Buoyant
Thanks to its porous nature and the air that becomes trapped in the many bubble holes, pumice is the only rock that floats. Pumice will eventually get waterlogged and sink. Leave your pumice in the water and it will slowly disintegrate.
Lightweight
Because pumice is so lightweight, it's used to make light, strong construction materials like concrete block and concrete. Three-quarters of the pumice and fine grains of pumice called pumicite go into construction materials, according to the Mineral Information Institute.
Color
If your pumice is light gray, it's high in silica and low in iron and magnesium, according to the Mineral Information Institute. But pumice also occurs naturally in yellow, brown and black, and may contain crystals of other minerals like feldspar, pyroxene and zircon.
Abrasive
Pumice is abrasive but gentle enough that it can be used to scour away calluses and dried skin, if you're careful. Pumice is sold in its natural irregularly shaped form, carved into small blocks or ground down and reformed in other shapes for use as an exfoliator. Pumice may also be used to very carefully scour away persistent stains on hard surfaces, although it can damage the surface you're working on if you don't use extreme care.
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