All forms of Obsidian are amorphous and in the mineral class of silica oxides. They are formed from volcanic flows when lava that was rich in silicic acids was cooled very quickly (flowed into a lake or the ocean). The result of this created a mass that is very rich in minerals, and sometimes has small gas inclusions. Obsidian can not technically be classified as a crystal as it was cooled too quickly to form crystal lattices like many other stones we use for healing.
Its coloring can ranges from a pale yellows, blues and greens to deeper reds and browns and finally as black. It should be noted that much of the brighter colored obsidians (blues, greens, some reds and yellows) in the market place have been re-formed by man through a heating process turning them into smoother tumbled shapes for easier handling as the edges on raw obsidian are quite shart which made them excellent arrow heads for ancient peoples. While perhaps not as zippy, energetically speaking after their reformed help from man, they still contain all the goodness built in from their original creation. So, they are just a bit slower acting over a longer period of time.