* Granite is an igneous rock that cools slowly underground. This slow cooling allows the minerals to grow into large crystals, resulting in the characteristic large grains.

* Obsidian is a volcanic glass formed when lava cools very quickly. It has no time to crystallize, giving it a smooth, glassy texture.

* Basalt is another volcanic rock, but it cools more slowly than obsidian. This results in smaller crystals than obsidian, but still much smaller than granite.

* Pumice is a lightweight, porous volcanic rock formed when lava containing gas bubbles cools quickly.

Let me know if you have any other questions!