Cooling rate of Pumice ,Obsidian and Granite and crystal size of Pumice ,Obsidian and Granite

Pumice and Obsidian are extrusive igneous rocks which cools rapidly. Obsidian do not form crystals. Pumice contains holes where gas bubbles are formed.
 
Granite is an intrusive igneous rock which cools down cools down slowly. Its crystals are visible with naked eye.