Obsidian is volcanic glass rather than a mineral, and because there is no crystal structure to interrupt the surface, it takes the sharpest polish of any common carving material. That is why small detailed figures work so well in it.
The trade-off is that glass chips. The edges of a carving are the vulnerable part, so keep it from swinging into the windscreen or the mirror stem. Genuine volcanic obsidian sometimes shows faint flow banding under strong light; man-made black glass usually shows small round bubbles instead.