Here's why:

* Composition: Obsidian is a volcanic glass, formed by rapidly cooling lava. It's primarily composed of silica (SiO2), but lacks the crystalline structure of granite.

* Formation: Granite is an intrusive igneous rock, meaning it forms when magma cools slowly beneath the Earth's surface. Obsidian, on the other hand, forms extrusively from lava that cools quickly on the surface.

* Texture: Granite is known for its coarse-grained texture with visible crystals. Obsidian is smooth and glassy due to its lack of crystal formation.

While both obsidian and granite are igneous rocks, they differ significantly in their composition, formation, and texture.