A polished obsidian disc on a stand. Strictly speaking it isn't a mineral at all.
Minerals have an ordered internal structure, atoms stacked in a repeating pattern. Obsidian has none. It forms when lava high in silica cools so fast that the atoms never get the chance to organise themselves, and it sets as glass instead, frozen mid-arrangement. That's why every other listing here has a crystal system and this one says none.
The disorder is the reason it works as a mirror. There are no crystal boundaries to scatter light at, so a worked surface goes properly flat and reflects cleanly. It's also why obsidian breaks in smooth curved shells rather than along flat planes, and why a fresh edge on it can come off sharper than a surgical blade.
Mohs 5 to 5.5, so the polished face marks more easily than quartz does. Dust it before wiping, and use a soft dry cloth. Chipped obsidian edges are genuinely sharp, so handle a damaged piece with care.
Mineral: Obsidian, a volcanic glass
Crystal system: None, obsidian is amorphous
Origin: Mexico
Hardness: 5 to 5.5 on the Mohs scale; the polished face scratches easily and shows marks
Treatment: None
Dimensions: 12.3 x 8.35 x 21.5 cm
Weight: 335 g Note: Supplied on a stand
You will receive the exact piece photographed.