Here's why:

* Glassy: Obsidian forms when lava cools rapidly, preventing crystals from forming. This results in a smooth, glassy texture.

* Fine-grained: The rapid cooling doesn't allow for the formation of large crystals, resulting in a very fine-grained structure.

* Extrusive: Obsidian is formed from lava that erupts onto the Earth's surface, making it an extrusive igneous rock.

Other possibilities:

While obsidian is the most common, some other glassy extrusive rocks could be:

* Rhyolite: Can be glassy in some forms, but it usually has a more pronounced crystalline texture.

* Tachylite: A very dark, glassy volcanic rock similar to obsidian but formed from basaltic lava.

To confirm the identification, consider:

* Color: Obsidian is typically black or dark brown, but can also be green, red, or iridescent.

* Fracture: Obsidian fractures in a smooth, curved manner, often described as "conchoidal."

* Composition: A chemical analysis can confirm the specific composition and distinguish between different types of glassy rocks.