Here's why:

* Extrusive rocks form when magma (molten rock) erupts onto the Earth's surface as lava and cools quickly. This rapid cooling doesn't allow for the formation of large crystals, resulting in a glassy texture.

* Intrusive rocks form when magma cools and solidifies beneath the Earth's surface. This slower cooling process allows for the formation of larger crystals.

Obsidian's glassy texture and lack of visible crystals are telltale signs of its rapid cooling, which is a characteristic of extrusive rocks.