A single bead on a cord is the cheapest form of stone pendant, and it works because the shape gets out of the way of the material. Obsidian polishes to a surface with no visible texture at all, unlike a crystalline stone.
The bead is natural volcanic obsidian, checked for the small round bubbles that mark man-made black glass. The cap is brass-plated over an alloy core rather than solid, which is normal at this size and stated here rather than glossed over.