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An obsidian rock is an extrusive igneous rock that forms when a volcano erupts and has no time to crystallize, so it has a glassy texture. Obsidian rocks are generally black i â¦ n color.o rock is completely impermeable, but some porous rocks like shale and pumice are mostly impermeable, because â¦ 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. 2 people found this useful.
Best Answer: Permeability is a function of the amount of porespace in the rock and how interconnected the pore space is: granite and other intrusive igneous rocks have very interlocking grain structure and little pore space so low permeability.n metamorphic rocks the alignment of the platy minerals serves to further destroy permeability. Of the sedimentary rocks, shale or slate is least permeable (and thus forms good seals to oil and gas reservoirs), and sandstones and limestones most permeable.