Here's why:

* Igneous rocks are formed from the cooling and solidification of magma (molten rock below the Earth's surface) or lava (molten rock erupted onto the Earth's surface).

* Extrusive igneous rocks form when lava cools and solidifies quickly on the Earth's surface. This rapid cooling prevents the formation of large crystals, resulting in a glassy texture.

Obsidian is a volcanic glass formed when felsic lava (rich in silica) cools very quickly. This rapid cooling doesn't allow crystals to form, resulting in a smooth, glassy texture.