Igneous Rock Key

Crystal Size Composition Rock
Phaneritic Ultramafic Peridotite
Mafic Gabbro
Intermediate Diorite
Felsic Granite

Crystal Size Composition Rock
Aphanitic Mafic Basalt
Intermediate Andesite
Felsic Rhyolite

Crystal Size Composition Rock
Porphyritic Mafic Basalt Porphyry
Intermediate Andesite Porphyry
Felsic Rhyolite Porphyry

Texture Color Rock
Glassy Black, brown, red Obsidian
(containing a great number of bubble holes formed by escaping gas)
Dark; heavy Vesicular Basalt
Dark; light weight Scoria
Light Pumice
(ash and broken pieces of lava and glass formed into rock)
Any color Tuff or Breccia