What is "natural glass"

Usage examples of "natural glass".

Obsidian: Also called natural glass or volcanic glass, obsidian is a hard, glossy, and black ornamental stone.

Crystals of opaque quartz, adorned with limpid drops of natural glass suspended to the roof like lusters, seemed to take fire as we passed beneath them.

That would explain why the Moon seems to be covered with glassy bits, although natural glass is very uncommon on Earth.

And yet, as they looked about them, each in his own separate way, they saw that in still other places it was slick and polished, like a natural glass in which they might see themselves reflected.

This had probably been done centuries ago, for the walls and ceiling were crudely hacked from rough volcanic stone that glittered with chunks of obsidian, the natural glass that must have fused when this tall peak was an active volcanic cone.