* Formation: Obsidian forms when lava cools and solidifies very quickly, often with little to no time for crystals to form. This rapid cooling happens when lava erupts onto the Earth's surface (extrusive setting).

* Texture: Obsidian is characterized by its glassy, smooth texture, which is a result of the fast cooling process that prevents the formation of mineral grains.

* Composition: Obsidian is typically rich in silica (SiO2), which gives it a dark color, often black, but can also be brown, green, or even red. This composition is typical of felsic lavas, which are commonly associated with extrusive settings.

In contrast to intrusive igneous rocks, which cool slowly underground, obsidian's rapid cooling and glassy texture are definitive signs of an extrusive origin.