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My bet's on black basalt as being more likey. Obsidian is actually not all that strong; it is, after all, basically volcanic glass. It takes one hell of an edge when chipped (an obsidant knife blade, in fact is sharper than all but the most highly honed steel blades) but it is the one of the last stones you'd want to try and build with. you couuld assumedng you had chunks big enough polish it to mirror smoothness but it would take an incredilbe amount of time. Leaving the rough edges on the outside sounds good (it would basically mean you had a wall covered with flesh shearing edges) until you realize that should the walls be heated (as they migh by someone lighting fires around the bottom, during a seige) the stones wouln't just crack, they would shatter showering your enemies and your own tropps with a rain of razor sharp shrapne. Not to mention that obsidian has a lot of internal bubbles of its own which could pop and crack in these same temperatures.) Basalt on the other hand is very strong and study, weathering better than even granite. True it isnt neccerily shiny, but it will take a good polish if it is dressed and it can be sort of sparkly if it has the right crystals in it.
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