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Pumice:
Famous as the "rock that floats", pumice is really glass just like obsidian. It forms when magma cools and solidifies very rapidly on the earth's surface (extrusive). This rapid cooling prevent crystals from forming (pumice is non-crystalline) and gives it a glassy texture.
The difference between pumice and obsidian is that the magma that forms pumice contains a great deal of dissolved gas. When this magma reaches the surface it's just like soda exploding from a can of soda. The gas escapes, the magma expands into a kind of rock foam and the foam solidifies into pumice.
The gas bubbles (vesicles) remain in the rock making it very rough and giving it a 'vesicular texture'. This is also why pumice is very low density (all those little bubbles) and why some samples may actually float in water. Samples vary according to the minerals they contain.
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