Spheres are the hardest common form in stonework — a ball must be ground evenly in every direction, and any deviation shows immediately when the object is rotated under a light. In obsidian the difficulty is compounded because glass chips rather than wears, so a single slip at the final polish ruins the piece. That is most of what you are paying for here.
Obsidian has essentially no internal structure, which is why it polishes to this level. A quartz sphere of the same size would be hazier and considerably more expensive for the same finish, and a sphere sold as "black crystal" at a low price is usually glass.
One safety note we take seriously: a clear or dark sphere near a window can focus sunlight like a lens. Keep this on a shelf away from direct sun rather than in front of one.