Here's why:

* Formation: Obsidian forms when lava cools and solidifies very quickly, with little to no crystal growth. This rapid cooling traps the silica molecules in a glassy, amorphous state.

* Appearance: Obsidian has a smooth, glassy texture and a distinctive, dark color ranging from black to brown, green, or red.

* Composition: Like glass, obsidian is primarily composed of silica (silicon dioxide).

So, while it's not technically glass in the same way we use the term for everyday objects, obsidian's formation and properties make it a natural, volcanic glass.