Obsidian is the name for an igneous rock with a glassy texture. Vesicles form from escape of gases from a cooling magma. Sometimes magma cools and solidifies around gas bubbles, forming the rounded openings. Dark-colored vesicular igneous rocks are called scoria , or the term is used as a modifier to a rock name, as in scoriaceous basalt . A light-colored rock with many tiny openings is called pumice .
Pumice forms from a frothy, silica-rich magma.
They are dark-colored, often dark green, dark brown, or nearly black. If the igneous rock is aphanitic, individual minerals can't be identified from a hand sample. In this case, the composition can be gaged by the color of the rock. Igneous rock classification We will use the simplified classification scheme shown below to name the igneous rocks. The classification scheme is based on texture and mineral composition.