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Obsidian is a glass like material formed by very rapid cooling of molten rock. It occurs during extrusive igneous events (the magma is sent to the surface very quickly). Most of this magma will manage to form minerals, just really small ones and will become a rock. However a small amount of the melt will not have enough energy/time to form true minerals and will cool as micro minerals and a glassy material. Like glass, the resultant obsidian will have no long term order: this simply means that they don't form true crystallune structures. The lack of long range order leads to concodial fracture, which is why glasses and flints are sharp when broken. Some kinds of concoidal fracture are sharper than others -- broken obsidian will definitely do more damage than broken flint. 

The obsidian on the stall at the moment is banded flow obsidian. When held to the light, you will see obvious darker and lighter bands within most of the samples. This is partly to do with temperature changes as the melt is rising, but also to do with differentials in heat flow as the lava is cooling and the rock is forming on or near the surface. The darker bands contain larger concentrations of microminerals.

The presence of microminerals is a big deal, its why obsidian is not always black and it might be an explanation for things like sheen obsidian.