Thanks for the Sansa Clip suggestion. It looks like I'll definitely have to buy a compatible device to listen to audible books because audible will not play on just any MP3 player. They're as protectionist and anti-competitive as Australian publishers.
No rock is completely impermeable, but some porous rocks like shale and pumice are mostly impermeable, because the pores are not interconnected, thus not allowing fluids or gases to pass through them. This is why shale makes a good cap-rock, and pumice floats instead of sinking.
The pumice stone is extremely rough and very porous and surprisingly light when picked up. This is the only stone that will actually float on water until becoming waterlogged, at which point it will sink. The mineral makeup of pumice depends on the type of magma that composes the pumice foam. Basalt Minerals Basalt is a gray to black fine-grained volcanic rock that is often the origin of pumice.