The only rock that floats on water is "Pumice"

Pumice is vesicular or has gas pockets, or in easier terms, holes on the outside of the rock. This makes the rock less dense so it will float on water.


Pumice is a vesicular volcanic rock that is commonly light enough to float in water. It typically has a chemical composition similar to rhyolite (or its plutonic counterpart, granite), although magma of virtually any composition can form pumice. The term vesicular refers to the presence of vesicles, or irregularly shaped cavities, that produce a sponge-like or bubbly texture and very low density in volcanic rocks.

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