Obsidian is not a mineral in the usual sense: it is glass, formed when lava cools so quickly that atoms do not have time to arrange themselves into crystals. That is why a polished obsidian sphere is a single flat black with no grain — unlike granite or marble, there is no internal structure to catch the light.
Not every black stone sold as obsidian is obsidian. Much of the cheap trade material is man-made black glass, which is denser and typically has small round bubbles. Real volcanic obsidian usually shows faint flow lines and occasionally a rainbow sheen under direct light.