Within the obsidian, fine layers of cristobalite—a high-temperature silica—or gas bubbles reflect light, producing a silver sheen. As with golden obsidian, the internal structure determines how the stone meets light.

Formed only when lava cools within a specific range. Too fast or too slow, and the lustre does not appear.

Found in volcanic regions of Mexico and Brazil. The conditions for silver to emerge from black amorphous glass are extremely narrow. A stone carrying the colour of moonlight—though the light source is inside.