How do igneous rocks differ from each other.

Although hand specimen identification is some what of an art; igneous rocks are classified using two critiria:
•Texture
•Chemical and mineral composition
Most of the time the rock name, such as basalt or granite, reflects the chemical composition. The minerals and textures are used as adjectives that modify the rock name. You might see a name like an olivine basalt or a quartz porphyry rhyolite.