Igneous rock
Obsidian, pumice and scoria
Glassy volcanic rocks: obsidian is smooth volcanic glass with a glassy luster; pumice is highly vesicular felsic glass that can float; scoria is highly vesicular mafic glass.
In thin section
Volcanic glass is isotropic (black in XPL).
More photos and thin sections
Alex Strekeisen: Obsidian · Alex Strekeisen: Pumice · Alex Strekeisen: Basaltic scoria
Other igneous rocks
Granite · Diorite · Gabbro · Peridotite and dunite · Rhyolite · Andesite · Basalt · Komatiite
Sources
Panchuk, Physical Geology 7.3: Classification of Igneous Rocks