Judging quality
The sheen sweeps across the surface as the stone tilts. If the shine sits on the surface rather than moving through it, the piece is coated.
Volcanic glass
Obsidian with a metallic golden or silver shimmer produced by a layer of aligned microscopic gas bubbles reflecting light in unison.

The sheen sweeps across the surface as the stone tilts. If the shine sits on the surface rather than moving through it, the piece is coated.
Glass; avoid abrasives.
Polished obsidian mirrors were used for scrying in both Mesoamerica and Elizabethan England — John Dee's mirror in the British Museum is Mexican obsidian.
Cultural history, recorded as tradition rather than as medical or scientific claim.
See it in person
Both rooms are open six days a week, and there is always someone happy to pull pieces out of the case so you can compare them under the light.