Volcanic glass is a type of igneous rock that forms when molten LAVA cools quickly. Obsidian, with 8 letters, is a type of volcanic glass that is usually black in color and has a glassy texture. LAVA, with 4 letters, is molten rock expelled by a volcano during an eruption. MICA, with 4 letters, is a group of minerals that form thin, flexible sheets. Pumice, with 6 letters, is a type of volcanic glass that is light in color and has a porous texture.